Lithium-rich layered oxides (LrLOs) deliver extremely high specific capacities and are considered to be promising candidates for electric vehicle and smart grid applications. However, the application of LrLOs needs further understanding of the structural complexity and dynamic evolution of monoclinic and rhombohedral phases, in order to overcome the issues including voltage decay, poor rate capability, initial irreversible capacity loss and etc. The development of aberration correction for the transmission electron microscope and concurrent progress in electron spectroscopy, have fueled rapid progress in the understanding of the mechanism of such issues. New techniques based on the transmission electron microscope are first surveyed, and the applications of these techniques for the study of the structure, migration of transition metal, and the activation of oxygen of LrLOs are then explored in detail, with a particular focus on the mechanism of voltage decay.Download high-res image (276KB)Download full-size image

•Sheet-like SiOx are first prepared by delaminated siloxene which derives topotactic transformation of layered CaSi2.•Silicon suboxides exhibit Si nanodomains confined in amorphous SiO2.•The delicate designed carbon coated SiOx anodes deliver superior cycling performance and rate capability.SiOx owing to high reversible capacity and moderate volume expansion has been attracting a lot of attention as state-of-the-art anodes for the next generation of Li-ion batteries. However, poor cycling performance and poor rate capability, respectively associated with detrimental volume expansion and insulative amorphous SiO2, are still challenging issues which need to be addressed for the actual employment as anodes for Li-ion batteries. In this regard, here we design, synthesize, characterize and test carbon coated sheet-like SiO1.1 nanocomposites formed by Si-nanodomains confined inside amorphous SiO2 (nano-Si/a-SiO2). As a proof-of-concept, we achieve sheet-like SiOx nanocomposites via in-situ transformation of delaminated siloxene. In particular, self-prepared siloxene with oxygen-inserted Si6 rings terminated with H and OH ligands is prepared by delamination of CaSi2 in dilute HCl. Importantly, the resulting carbon coated nano-Si/a-SiO2 material shows enhanced reaction kinetics and structural stability leading to 946 mAh g−1 capacity at 0.15 A g−1. Intriguingly, 38.0% (~360 mAh g−1) of the maximum capacity is maintained even at 7.5 A g−1, corresponding to a remarkable less than 3 min charge/discharge time. Finally, the electrode shows merely 24% of volume expansion and minor cracks with capacity retention of 92% after 300 cycles at 7.5 A g−1.We introduce a strategy to prepare sheet-like Si/SiO2 nanocomposites from siloxene. Siloxene was delaminated from layered CaSi2. After CVD process, sheet-like Si/SiO2@C showing Si nanodomains well confined within amorphous SiO2 matrix exhibits superior electrochemical performance as anode for Li-ion batteries.Download high-res image (441KB)Download full-size image

Three dimensional (3D) porous silicon/reduced graphene oxide (Si/rGO) composites with typical networks have suffered damage during electrode preparation, which evidently affects the cycle and rate capabilities of Si/rGO anodes. Here, a controllable evaporation dry method is proposed to fabricate structure-preserved 3D porous Si/rGO anode materials by tuning the pore size distribution of the networks. As a result, after evaporation drying for 3.5 h, the optimal sample of 3D porous Si/rGO anode (denoted as Si–G-3.5) with a pore size of ∼500 nm could preserve its 3D network during the electrode preparation process. While the structures of Si/rGO composites with different drying times (denoted as Si–G-0, Si–G-2.5 and Si–G-4) failed to be preserved. Consequently, The Si–G-3.5 anode exhibits a high reversible specific capacity of 1563 mA h g−1 at 50 mA g−1, 90% capacity retention after 100 cycles and superior rate capability (955 mA h g−1 at 2 A g−1).

The cyclability of lithium-ion batteries (LIBs) is often affected by the components of the solid electrolyte interphase (SEI) layer which is generated from electrochemical decomposition of electrolyte. Here, lithium difluorophosphate (LiPO2F2) is studied in this work. When 1.6 wt% LiPO2F2 additive is incorporated into the reference electrolyte, the capacity retention of graphite/Li half-cell is increased from 82.53% to 98.04% and the capacity retention of LiCoO2/Li half-cell is increased from 89.60% to 97.53% after 160 cycles. Electrochemical impedance spectroscopy (EIS) indicates that the SEI layer containing LiPO2F2 can decrease the surface impedance of cells in the last stage cycle. In situ atomic force microscopy (AFM), DFT calculations and X-ray photoelectron spectroscopy (XPS) results show that LiPO2F2 is deposited on the surface of both LiCoO2 and graphite electrodes, which effectively protects the graphite anode and suppresses the degradation of the cathode during the long-term cycling of LIBs.

As rechargeable Li-ion batteries have expanded their applications into on-board energy storage for electric vehicles, the energy and power must be increased to meet the new demands. Li-rich layered oxides are one of the most promising candidate materials; however, it is very difficult to make them compatible with high volumetric energy density and power density. Here, we develop an innovative approach to synthesize three-dimensional (3D) nanoporous Li-rich layered oxides Li[Li0.144Ni0.136Co0.136Mn0.544]O2, directly occurring at deep chemical delithiation with carbon dioxide. It is found that the as-prepared material presents a micrometer-sized spherical structure that is typically composed of interconnected nanosized subunits with narrow distributed pores at 3.6 nm. As a result, this unique 3D micro-/nanostructure not only has a high tap density over 2.20 g cm–3 but also exhibits excellent rate capability (197.6 mA h g–1 at 1250 mA g–1) as an electrode. The excellent electrochemical performance is ascribed to the unique nanoporous micro-nanostructures, which facilitates the Li+ diffusion and enhances the structural stability of the Li-rich layered cathode materials. Our work offers a comprehensive designing strategy to construct 3D nanoporous Li-rich layered oxides for both high volumetric energy density and power density in Li-ion batteries.Keywords: 3D nanoporous; cathode materials; lithium-ion batteries; lithium-rich oxides; power energy density; volumetric energy density;

•Concentration-gradient LiMn0.8Fe0.2PO4 is constructed through solvothermal method.•The Mn dissolution of concentration cathode material is suppressed.•The rate capability of this concentration material is improved.•The cycle stability is improved, especially at elevated temperature.It is a great challenge to combine good cycling performance with high rate capability for LiMn1–xMxPO4 cathode materials owing to the Mn dissolution upon cycling and its low electronic/ionic conductivity. Here, we report a novel concentration-gradient structure of LiMn0.8Fe0.2PO4 material constructed by solvothermal treatment. This material shows a linear increase of Mn concentration from the edge to the particle centre, but the inverse trend for Fe concentration, which leads to the formation of Mn-rich phase in bulk and Fe-rich phase at surface. The Fe-rich phase effectively suppresses the corrosion from the electrolyte that minimizes the Mn dissolution and also improves the electronic/ionic conductivity of the surface that decreases the cathode/electrolyte interface resistance. Consequently, this concentration-gradient material achieves superior capacity retention with 98% after 50 cycles at 1 °C even at elevated temperature, and also exhibits an excellent rate capability with the reversible capacity of 130 mA h g−1 at 5 °C rate. These results suggest that the concentration-gradient LiMn0.8Fe0.2PO4 is an ideal type of cathode material for high performance Lithium ion batteries.

•Copper sulfide (CuS) nanorods were successfully synthesized.•A facile sol–gel method without using template and complicated treatment was used.•The as-prepared CuS nanorods as anode achieve great electrochemical performance.Copper sulfide (CuS) nanorods with the size of sub-10 nm are synthesized via a facile sol–gel method without post–thermal treatment. The as-prepared CuS nanorods are characterized by X-ray diffraction, transmission electron microscope, and energy dispersive X-ray spectroscopy as hexagonal covellite CuS. The as-prepared CuS nanorods utilized as anode material exhibit a high reversible capacity and excellent cycling stability up to 250 cycles, as well as high Coulombic efficiency. The unique structure of the CuS nanorods should be responsible for their excellent electrochemical performance.

•Small B3+ is doped into tetrahedral sites in Li-rich layered oxides.•Rietveld refinement indicates enlarged cell parameters of B doped samples.•Doped B blocks the migrated path of the transition metal ions.•B doped materials manifest greatly enhanced capacity and voltage stability.Migration of transition metal (TM) ions to tetrahedral sites plays a crucial role on structural transformation and electrochemical behaviors for Li-rich layered oxides. Here, incorporating small B3+ in the tetrahedral interstice is employed to block the migration channel of TM ions and stabilize the crystal structure. Benefiting from their good structural stability, Li-rich layered materials with B-doped Li1.198Ni0.129Co0.129Mn0.535B0.01O2 and Li1.196Ni0.127Co0.127Mn0.529B0.02O2, exhibit excellent cycling performance and voltage stability. After 51 cycles at 0.2 C, 1 mol.% boron incorporated sample can deliver 211 mAh g−1 with capacity retention of 89.9%, which is much higher than that of the undoped sample of 177 mAh g−1 with the retention of 79.2%. Moreover, the declined voltage per cycle decreases from 3.6885 mV to 2.7530 mV after 2 mol.% boron doping. XRD patterns after extended cycling verified the suppression of the structural transformation by the incorporation of boron.

•The durable carbon layer is constructed by CVD coating method, successfully.•Mn dissolution of treated cathode material is effectively suppressed.•Long-term cycling performance of the treated cathode material is improved.•The rate capability of the treated material is enhanced.LiMn0.8Fe0.2PO4 is becoming one of the most promising cathode materials for lithium ion batteries. However, the capacity suffers from a loss during long-term cycling, which is directly associated with Mn dissolution due to the disproportionation reaction of Mn3+. Here, we report a chemical vapor deposition (CVD) approach to modify LiMn0.8Fe0.2PO4 particles with carbon so as to minimize Mn dissolution from cathode. The deposited carbon layer not only protects LiMn0.8Fe0.2PO4 cathode from electrolyte corrosion, but also enhances the electronic/ionic conductivity owing to its higher graphitize degree. As a consequence, the electrochemical performances have a significant improvement. The capacity retention achieves 96% after 450 cycles at 1 C at room temperature (25 °C). Even at elevated temperature (55 °C), the capacity retention also reaches at 97 % after 50 cycles at 1 C rate, which is much higher than that of untreated sample (89%). Hence, the cathode material based on LiMn0.8Fe0.2PO4 encapsulated with durable carbon by CVD method represents a promising strategy for developing its long-term cycling performance through suppressing Manganese dissolution.

•Micron Li-rich oxides have been synthesized by a new two-step method.•Micron particles result in high pellet density and good cycling performance.•High capacity of the micron Li-rich oxide was achieved after Na2S2O8 treatment.Li-rich layered oxides with micro-sized primary particles usually exhibit higher pellet density and better cycling performance. However, it is often at the expense of high reversible capacity. Here, we reported a simple strategy through a new chemical lithiation with micro-sized spinel-precursors and Na2S2O8 surface treatment to obtain micro-sized particles in the Li-rich Li1.172Ni0.135Co0.135Mn0.539O2 oxide without compromising its discharge capacity (260 mAh/g at 0.1C). Benefited from its larger particles and lower specific surface area, the obtained micron Li-rich layered material manifests its higher pellet density (3.18 g/cm3) and better cycling performance in comparison with the nano-sized Li-rich layered material. After 100 cycles at 0.1 C, it can still deliver a capacity of 192 mAh/g with retention of 74%, much higher than 167 mAh/g with retention of 67% of the nano-sized sample. Moreover, the micron Li-rich layered material also exhibits good rate performance with a capacity of 174 mAh/g at 2C benefited from the 3D Li+ insertion/extraction channel of its surface spinel content. These results may provide a new insight on improving pellet density of high-performance Li-rich layered cathode materials.

Silicon, as a next generation anode material, suffers from low electronic conductivity and large volume changes during the lithiation/delithiation process, resulting in very large capacity fading upon cycling. Herein, we design a novel sandwich-structured Si/C electrode formed between two conductive carbon layers. In this configuration, the bottom carbon layer functions as a buffer layer to increase the adhesion to the Cu foil and to avoid peeling-off of the active materials, whereas the top carbon layer on the electrolyte side serves as a barrier layer to prevent the electrode surface from cracking and delaminating. As expected, the sandwich-structured Si/C electrode delivers a high reversible capacity of 1230 mA h g−1 at current density of 150 mA g−1 and exhibits excellent cycling stability without obvious capacity decay after 70 cycles. This simple and effective design would be a promising approach to obtain high performance and cost-effective Si anodes on a large-scale, especially for industrial manufacturing of high energy density Li-ion batteries.

The Li-rich layered oxides are attractive electrode materials due to their high reversible specific capacity (>250 mA h g−1); however, the origin of their abnormal capacity is still ambiguous. In order to elucidate this curious anomaly, we compare the lattice oxygen oxidation states among the Li-rich layered oxide Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3 and LiNi0.5Co0.2Mn0.3O2, the two components in Li-rich layered oxides, and the most common layered oxide LiCoO2 before and after initial charge–discharge. For simplicity, we employ chemical treatments of NO2BF4 and LiI acetonitrile solutions to simulate the electrochemical delithiation and lithiation processes. X-ray photoelectron spectroscopy (XPS) studies reveal that part of lattice oxygen in Li1.14Ni0.136Co0.136Mn0.544O2 and Li2MnO3 undergoes a reversible redox process (possibly O2− ↔ O22−), while this does not occur in LiNi0.5Co0.2Mn0.3O2 and LiCoO2. This indicates that the extra capacity of Li-rich layered oxides can be attributed to the reversible redox processes of oxygen in the Li2MnO3 component. Thermogravimetric analysis (TGA) further suggests that the formed O22− species in the delithiated Li1.14Ni0.136Co0.136Mn0.544O2 can decompose into O2 at about 210 °C. This phenomenon demonstrates a competitive relationship between extra capacity and thermal stability, which presents a big challenge for the practical applications of these materials.

A composite membrane with an ultra-thin ion exchangeable layer is specially designed as a separator in lithium ion batteries with manganese-based cathode materials. The composite membrane features a Mn2+ capture function which originates from the ion exchanging process, especially at high temperature, and is proven to help to alleviate the capacity decay of lithium ion batteries effectively. The enhanced thermal stability, improved wettability and higher lithium ion transference number of the composite membrane further suggest its promising application in lithium ion batteries.

•Transition metal ion migration occurs at the potential above the activation energy.•The charge/discharge lower-limit voltages have no impact on voltage decay.•Voltage decay becomes severer with increased upper-limit voltages.•Holding at high voltages for a prolonged time also accelerates voltage decay.Voltage decay of Li-rich cathode material is caused by migration of transition metal (TM) ions and layered phase transformation to spinel phase. Here electrochemical studies demonstrate that charge/discharge voltage ranges have a strong impact on the voltage decay. For cells cycling in the charge/discharge condition of voltage range at 2.0–4.6 V, 3.2–4.6 V and 4.2–4.6 V, voltage decay phenomena occur to the same extent. While for the serial conditions of 2.0–4.2 V, 2.0–4.4 V, 2.0–4.6 V, 2.0–4.8 V and 2.0–4.6V + hold at 4.6 V for 5 h, voltage decay starts to occur at a typical voltage when the potential is high enough, and then phase transformation becomes severer with increased potentials, or prolonged time holding at high voltages. TM ions at high voltages have high enough energy, which can be denoted as “activation energy”, to stride across the transition state and migrate to Li vacancy, which results in spinel formation and voltage decay.

•Thiophene derivatives (THs) can serve as novel electrolyte additives for LIBs.•THs tend to be electro-polymerized on LiCoO2 cathode prior to the solvents.•A conducting polymer film can be formed using THs additives in the electrolyte.•The cycling stability of high voltage LiCoO2 is obviously improved using additives.Thiophene derivatives (THs) are examined as novel functional additives for improving the cycling performance of high-voltage LiCoO2. Our investigation reveals that 2,2′-Bithiophene (2TH) and 2,2′:5′,2′′-Terthiophene (3TH) can be electrochemically polymerize prior to the electrolyte solvent decomposition to form a protective layer of conducting polymer film on the cathode surface, which blocks off severe electrolyte decomposition at high voltages and, therefore, improves the cycling stability of high voltage LiCoO2 cathode. After 100 cycles at a high cutoff voltage of 4.4 V, the discharge capacity retention is 50% in the base electrolyte, in contrast, the LiCoO2 cathode cycled in the electrolyte containing 0.1 wt% 3TH displays a high capacity retention of 84.8% at 0.25 C rate. This work demonstrates that these thiophene derivatives have considerable potential as functional additives for the applications in high-voltage lithium-ion batteries.

To develop a kind of gel polymer electrolyte with high ion conductivity and good mechanical strength and thermal stability, a polyimide (PI) matrix-enhanced cross-linked gel separator is designed and fabricated by a simple dip-coating and heat treatment method. The PI nonwoven substrate provides high-temperature thermal stability for the gel separator and the crosslinked gel part yields enhanced affinity with the liquid electrolyte. Besides, the cross-linked polymer network could solve the issue of long-term durability of the composite separator in batteries. The gel separator shows better cyclability and rate capability than the traditional PP separator, implying a promising potential application in high-power, high-safety lithium ion batteries. The preparation process is compatible with the traditional manufacturing process of nonwoven membranes, and can be easily converted into continuous production on the industrial scale.

Electrode films fabricated with lithium-rich layered 0.3Li2MnO3–0.7LiNi5/21Co5/21Mn11/21O2 cathode materials have been successfully modified with ZnO coatings via a reactive magnetron sputtering (RMS) process for the first time. The morphology and chemical composition of coating films on the electrodes have been in deep investigated by transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS) characterizations. The results clearly demonstrate that ZnO film coatings are ultrathin, dense, uniform, and fully covered on the electrodes. The RMS-2 min (deposition time) coated electrode exhibits much higher initial discharge capacity and coulombic efficiency with 316.0 mAh g–1 and 89.1% than that of the pristine electrode with 283.4 mAh g–1 and 81.7%. In addition, the discharge capacity also reaches 256.7 and 187.5 mAh g–1 at 0.1 and 1.0 C-rate, as compared to that of 238.4 and 157.8 mAh g–1 after 50 cycles. The improved electrochemical performances of RMS-coated electrodes are ascribed to the high-quality ZnO film coatings that reduce charge transfer resistance and effectively protect active material from electrolyte oxidation.Keywords: initial coulombic efficiency; lithium-ion batteries; lithium-rich layered oxide cathode; reactive magnetron sputtering; zinc oxide coating;

•The surface structure transforms from layered to spinel during heat treatment.•The surface spinel undergoes an internal structural evolution at high temperature.•The electrochemical performances are obviously improved after heat treatment.•The mechanism of structural transformation is discussed.A surface modification strategy through soaking in Na2S2O8 aqueous solution and then annealing has been developed for Li-rich layered cathode materials for Li-ion batteries. The modified materials have a significant improvement on electrochemical performances. The initial discharge capacity increases from 257 to 285 mAh g−1, and the initial coulombic efficiency increases from 85.4% to 93.2% in the voltage rang of 2.0–4.6 V. The electrochemical enhancement mechanism has been revealed by detailed investigations on the surface structural conversion of the material. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma-atomic emission spectrometry (ICP) confirm that Na2S2O8 oxidizes lattice oxygen to formal O22− species and the corresponding Li+ is extracted from the material surface. On the subsequent annealing, the formal O22− species turn to O2 and release from the particle surface. The increased oxygen vacancies induce structural rearrangement and lead to the phase transition from layered (R-3m or C2/m) to spinel (Fd3m) at the particle surface, which is supported by X-Ray Diffraction (XRD) and high resolution transmission electron microscope (HRTEM). It is also found that the spinel phase increases with the increasing annealing temperature, and an internal structural evolution from LiM2O4-type spinel to M3O4-type spinel takes place at the same time.

•The ICE for Li-rich layered oxides is significantly dependent on the testing temperature.•The lithium intercalation into MnO2-like component greatly dominates the ICE.•The ICE reproducibly approaches 92% when discharged at 50 °C regardless of charging temperature.In this study we report on the temperature dependence of the initial coulombic efficiency (ICE) in Li-rich layered Li[Li0.144Ni0.136Co0.136Mn0.544]O2 oxide, consisting of rhombohedral LiNi1/3Co1/3Mn1/3O2 and monoclinic Li2MnO3 component confirmed by SXRD and SAED. The electrochemical result shows that the ICE increases from 74.6 to 91.5% with the applied charging/discharging temperature from 0 to 50 °C, and it reproducibly approaches 92% when discharged at 50 °C, regardless of the applied charging temperature. From the dQ/dV plots, it is observed that the discharging temperature significantly influences the lithium intercalation into the MnO2-like component derived from Li2MnO3 component, i.e. the discharge process determines the ICE. This phenomenon indicates that the lithium re-intercalation into the MnO2-like component appears to be the most important factor.

•The Fe-doping LiMnPO4 nanomaterials were synthesized by a solvothermal method.•The particle morphology could be controlled simply by adjusting the pH values of precursor suspensions.•The nanoplates along with [010] crystallographic axis with 20–30 nm could deliver the largest discharge capacity.•Fe doping could significantly increase the initial reversible capacity.The Fe-doping LiMnPO4 (LiMn1−xFexPO4, x ≤ 0.5) nanomaterials are solvothermally synthesized in a mixed solvent of water and polyethylene glycol (PEG). The particle morphology can be controlled simply by adjusting the pH values of precursor suspensions. Electrochemical test shows that LiMn0.9Fe0.1PO4 nanoplates with a thickness of 20–30 nm could deliver the largest discharge capacity, which is attributed to the fast Li+ diffusion in the diffusion path of [010] crystallographic axis along the short radial direction of the nanoplates. It is demonstrated that Fe doping could significantly increase the initial reversible capacity, cycle performance and rate capability. The first discharge capacities of Fe-doped LiMnPO4 are all above 150 mAh g−1 at the discharge rate of 0.05 C. Especially, LiMn0.5Fe0.5PO4 delivers 100% capacity retention with the reversible capacity of 147 mAh g−1 at the discharge rate of 1 C, and losses only about 23.4% capacity with the discharge rate varying from 0.1 C to 5 C. The variation of energy density predicts that LiMn0.5Fe0.5PO4 shows the potential application for high-power devices.

•As-prepared carbonate precursor possesses concentration gradient.•Spherical LiNi0.5Mn1.5O4 material is core–shell structure.•The batteries have superior rate capability and high-temperature cycle performance.•Excellent electrochemical properties come from its unique core–shell structure.Spherical LiNi0.5Mn1.5O4 material with a core–shell structure is synthesized by a urea-assisted hydrothermal method followed by heat treatment with LiOH at high temperature. After the process of hydrothermal treatment, the carbonate precursor with a concentration gradient is produced, in a single spherical particle, the content of Ni in the surface is higher than that in the center while Mn has a reversal trend. LiNi0.5Mn1.5O4 synthesized through the hydrothermal route has a great improvement in cycling stability at elevated temperature and rate capability. The capacity retention can maintain at 95% after 30 cycles at 55 °C. Furthermore, it can deliver a discharge capacity of 118 mAh g−1 at a high rate of 10 C at room temperature. Such excellent electrochemical properties of LiNi0.5Mn1.5O4 can be ascribed to its unique core–shell structure and nano-size particle.

The electrochemical performance of the 18650 lithium-ion batteries for layered Li-excess oxide Li1.144Ni0.136Co0.136Mn0.544O2(LNCMO) cathode material and mesocarbon microbead (MCMB) anode material is investigated. The battery shows an excellent rate capability with the capacity of 227 mAh g−1 at 8 C-rate (the cut-off voltage is 4.5 V). Furthermore, it exhibits excellent cycle performance that the capacity retention over 300 cycles in the voltage ranges of 2.5-4.5 V (vs. MCMB) and at 0.2 C-rate is about 85%. Although the medium voltage of the battery greatly reduces during the first 30 cycles, it keeps stable in the following cycles. The mechanisms of the capacity fade and voltage decay are also studied based on energy dispersive spectrometry, X-ray photoelectron spectroscopy, charge-discharge curves, and dQ/dV plots.

•The (1-x)LiMnPO4·LixTix(PO4)δ samples were synthesized through a solid-state method.•The content of Ti additive greatly affects the electrochemical performance of LiMnPO4.•The sample synthesized with 10% Ti exhibits the best electrochemical performance.The (1-x)LiMnPO4·LixTix(PO4)δ (x = 0, 0.01, 0.05, 0.10, 0.15, 0.20) cathode materials are successfully synthesized through a solid-state method. The structures and electrochemical properties of the prepared samples have been characterized comprehensively. It is found minority phases containing LiTi2(PO4)3 and TiP2O7 were formed. The addition of Ti has obviously reduced the size of grains. Electrochemical tests indicate that the discharge capacities of LiMnPO4 samples can be significantly improved with the addition of Ti. Especially, the (1-x)LiMnPO4·LixTix(PO4)δ sample with x = 0.1 has the largest discharge specific capacity, which is more than 131 mAh g−1 at 0.05 C. And EIS tests1demonstrate that the 0.9LiMnPO4·Li0.1Ti0.1(PO4)δ sample has lower charge transfer resistance and higher diffusion coefficient than the pristine LiMnPO4 sample.

•A series of (1−x)LiMnPO4·xLi3V2(PO4)3/C were synthesized by solid-state method.•The ratios of LiMnPO4 to Li3V2(PO4)3 are correlated with the properties of samples.•The 0.6LiMnPO4·0.4Li3V2(PO4)3 exhibits the best electrochemical performance.A series of (1−x)LiMnPO4·xLi3V2(PO4)3/C (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1) composites cathode materials are successfully synthesized by solid-state method. The structures and properties of the composites have been studied with X-ray diffraction (XRD), scanning electron microscopy (SEM), elemental mapping, high resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS) and electrochemical measurements. XRD results reveal that the composites comprise Li3V2(PO4)3 phase and LiMnPO4 phase with a small amount of LiVP2O7 impurity. The electrochemical measurement results show that the ratios of LiMnPO4 to Li3V2(PO4)3 are correlated with the electrochemical performances of the composite materials. Among these composites, the 0.6LiMnPO4·0.4Li3V2(PO4)3 exhibits the best electrochemical performance, it can deliver specific capacity of 154 mA h g−1 at 0.05 C charge–discharge rate.

Solid solutions between Li[Li1/3Mn2/3]O2 and LiMO2 (M = Ni1/3Mn1/3Co1/3, Ni0.4Mn0.4Co0.2, Ni0.45Mn0.45Co0.1, Ni0.5Mn0.2Co0.3, and Ni0.5Mn0.3Co0.2) have been synthesized by a solid-state reaction method. The as-prepared Li1.2Ni0.13Mn0.54Co0.13O2, Li1.2Ni0.16Mn0.56Co0.08O2, Li1.2Ni0.18Mn0.58Co0.04O2, Li1.2Ni0.2Mn0.48Co0.12O2, and Li1.2Ni0.2Mn0.52Co0.08O2 solid solutions cathode materials can deliver discharge capacities of 267, 262, 253, 235, and 238 mAh g−1, respectively, at a charge/discharge current density of 25 mA g−1 in the voltage range of 2.5–4.7 V. These cathodes all have initial coulombic efficiencies larger than 80%, and show capacity loss less than 0.13% per cycle while cycling at 125 mA g−1 for 50 cycles. From Rietveld refinement results and electrochemical impedance spectra (EIS) analysis, it is found that the highest charge/discharge capacity values of Li1.2Ni0.13Mn0.54Co0.13O2 cathode with the lowest Ni content among them are attributed to the lowest cation mixing. While both the Li1.2Ni0.2Mn0.48Co0.12O2 and Li1.2Ni0.2Mn0.52Co0.08O2 cathode materials with higher Ni content exhibit better rate capabilities due to their lower charge transfer resistances and higher electrical conductivities than those of other samples.Highlights► Li-rich solid solutions have been synthesized by a solid-state reaction method. ► Li1.2Ni0.18Mn0.58Co0.04O2 and Li1.2Ni0.2Mn0.52Co0.08O2 are firstly reported. ► The higher Ni content is, the higher cation mixing is, and the lower resistance is. ► Cation mixing influences capacity, while resistance affects rate capability.

The Li-rich layered oxides are attractive electrode materials due to their high reversible specific capacity (>250 mA h g−1); however, the origin of their abnormal capacity is still ambiguous. In order to elucidate this curious anomaly, we compare the lattice oxygen oxidation states among the Li-rich layered oxide Li1.14Ni0.136Co0.136Mn0.544O2, Li2MnO3 and LiNi0.5Co0.2Mn0.3O2, the two components in Li-rich layered oxides, and the most common layered oxide LiCoO2 before and after initial charge–discharge. For simplicity, we employ chemical treatments of NO2BF4 and LiI acetonitrile solutions to simulate the electrochemical delithiation and lithiation processes. X-ray photoelectron spectroscopy (XPS) studies reveal that part of lattice oxygen in Li1.14Ni0.136Co0.136Mn0.544O2 and Li2MnO3 undergoes a reversible redox process (possibly O2− ↔ O22−), while this does not occur in LiNi0.5Co0.2Mn0.3O2 and LiCoO2. This indicates that the extra capacity of Li-rich layered oxides can be attributed to the reversible redox processes of oxygen in the Li2MnO3 component. Thermogravimetric analysis (TGA) further suggests that the formed O22− species in the delithiated Li1.14Ni0.136Co0.136Mn0.544O2 can decompose into O2 at about 210 °C. This phenomenon demonstrates a competitive relationship between extra capacity and thermal stability, which presents a big challenge for the practical applications of these materials.

A composite membrane with an ultra-thin ion exchangeable layer is specially designed as a separator in lithium ion batteries with manganese-based cathode materials. The composite membrane features a Mn2+ capture function which originates from the ion exchanging process, especially at high temperature, and is proven to help to alleviate the capacity decay of lithium ion batteries effectively. The enhanced thermal stability, improved wettability and higher lithium ion transference number of the composite membrane further suggest its promising application in lithium ion batteries.

To develop a kind of gel polymer electrolyte with high ion conductivity and good mechanical strength and thermal stability, a polyimide (PI) matrix-enhanced cross-linked gel separator is designed and fabricated by a simple dip-coating and heat treatment method. The PI nonwoven substrate provides high-temperature thermal stability for the gel separator and the crosslinked gel part yields enhanced affinity with the liquid electrolyte. Besides, the cross-linked polymer network could solve the issue of long-term durability of the composite separator in batteries. The gel separator shows better cyclability and rate capability than the traditional PP separator, implying a promising potential application in high-power, high-safety lithium ion batteries. The preparation process is compatible with the traditional manufacturing process of nonwoven membranes, and can be easily converted into continuous production on the industrial scale.