LiNi0.5Mn1.5O4−δ which possesses a high voltage of 4.7 V vs. Li+/Li and stable structure has been considered as a promising cathode material for high energy Li-ion batteries. In this study, well-crystallized LiNi0.5Mn1.5O4−δ nanorods with diameter of ∼50 nm and length of ∼500 nm were synthesized by a hydrothermal-solid state process followed by a lithiation reaction. It delivers ∼120 mA h g−1 discharge capacity under 30 mA g−1 at 3.5–4.9 V, and 90 mA h g−1 at 150 mA g−1. An unstable solid electrolyte interphase (SEI) film with thickness 2–10 nm was observed by transmission electron microscopy. The thick and unstable SEI layers are responsible for its low rate performance and low coulombic efficiency during cycling.

Lithium bis(fluorosulfonyl)imide (LiFSI) has been studied as conducting salt for lithium-ion batteries, in terms of the physicochemical and electrochemical properties of the neat LiFSI salt and its nonaqueous liquid electrolytes. Our pure LiFSI salt shows a melting point at 145 °C, and is thermally stable up to 200 °C. It exhibits far superior stability towards hydrolysis than LiPF6. Among the various lithium salts studied at the concentration of 1.0 M (= mol dm−3) in a mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, v/v), LiFSI shows the highest conductivity in the order of LiFSI > LiPF6 > Li[N(SO2CF3)2] (LiTFSI) > LiClO4 > LiBF4. The stability of Al in the high potential region (3.0–5.0 V vs. Li+/Li) has been confirmed for high purity LiFSI-based electrolytes using cyclic voltammetry, SEM morphology, and chronoamperometry, whereas Al corrosion indeed occurs in the LiFSI-based electrolytes tainted with trace amounts of LiCl (50 ppm). With high purity, LiFSI outperforms LiPF6 in both Li/LiCoO2 and graphite/LiCoO2 cells.Research highlights▶ Lithium bis(fluorosulfonyl)imide (LiFSI) has been studied as conducting salt for nonaqueous liquid electrolytes for lithium-ion batteries. ▶ Lithium bis(fluorosulfonyl)imide (LiFSI) exhibits far superior stability towards hydrolysis than lithium hexafluorophosphate (LiPF6) and does not release hydrogen fluoride (HF). ▶ Pure lithium bis(fluorosulfonyl)imide (LiFSI) does not corrode Al, and Al corrosion is induced by trace amounts of chloride (Cl−) impurities present in it. ▶ Lithium bis(fluorosulfonyl)imide (LiFSI) outperforms lithium hexafluorophosphate (LiPF6) for lithium-ion batteries.

A series of LiFe1−xMnxPO4/C materials with high Mn content (0.7 ≤ x ≤ 0.9) are synthesized by solid state reaction. The samples have mesoporous structure with an average pore size of 25 nm, particle size around 200–300 nm, crystalline size around 30 nm and specific areas around 50 m2 g−1. Their electrochemical performances are studied and the reversible capacity and rate performance decrease with the increase of Mn content. The redox potential of the Fe2+/Fe3+ and Mn2+/Mn3+ redox couple also shift accordingly. The overpotential value of the Mn2+/Mn3+ redox couple (80 mV) is close to that of the Fe2+/Fe3+ couple (60 mV) in all three compositions and shows a maximum (∼300 mV) in the regions of voltage transition.

LiFePO4 thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, D˜Li, were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average D˜Li values calculated from CV results were in the order of 10−14 cm2 s–1. The D˜Li values obtained by GITT, and EIS techniques were in the range of 10–14–10–18 cm2 s–1, 10–14–10–18 cm2 s–1, respectively. The D˜Li values obtained by the two methods show a minimum point at x ∼ 0.5 for Li1−xFePO4. However, the overpotential values of the LiFePO4 thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li1–xFePO4. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li1–xFePO4 which constitutes two phase, i.e., LiFePO4 and FePO4 in this region.Highlights► The average chemical diffusion coefficients of Li are 1.8 × 10−14–2.1 × 10−14 cm2 s–1 measured from CV. ► The minimum D˜Li values obtained from GITT and EIS are in contradiction with other experimental data. ► The deduction of D˜Li is highly dependent on the dE/dx values for GITT and EIS methods.

“LiNi0.5Mn1.5O4” is generally reported to crystallize in the spinel structure within two different space groups (Fd-3m and P4332) depending on the oxygen stoichiometry/ordering of the Ni/Mn cations. This paper presents a contribution to the study of these two phases in order to have a better insight on how the preparation method influence their electrochemical properties. The phases were synthesized by a ceramic route and thoroughly characterized using X-ray diffraction (XRD), FTIR and Raman spectroscopy. This latter technique, together with the obtaining of the electrochemical signature, allows discriminating between the types of spinel phases. Based on thermal in situ XRD measurements, we further propose a peritectoid transformation as an explanation for the appearance of a nickel based rock salt phase during the preparation. Finally, we discuss on the electrochemical deinsertion mechanism.Research highlights► High voltage spinel "LiNi0.5Mn1.5O4"are prepared and thoroughly characterized. ► The formation of the rock salt impurity is explained through a peritectoid phase transition and the electrochemical de-insertion mechanism was investigated. ► We present the electrochemical deinsertion mechanism and discuss it in relation with recent reports on the effect of structural strains.

The structural, electronic and Li diffusion properties of LiFeSO4F were analyzed by first-principles calculation under the DFT + U framework. The difference of the calculated lattice parameters and the reported data is within 3%. The redox potential of Fe2+/Fe3+ versus Li metal is 3.7 V, and phase separation of LiFeSO4F and FeSO4F is expected during Li extraction. Pure LiFeSO4F is an insulator with a band gap of 3.6 eV, while the band gap in the partially delithiated form Li1 − xFeSO4F is obviously smaller. A very low Li migration energy of 0.3 eV is required in the partially delithiated form Li1 − xFeSO4F, and the diffusion coefficient is estimated to be about 1.6 ⁎ 10−7 cm2 s−1.

Elastic band method has been used to calculate the activation energy for Li diffusion in the olivine structured LiFePO4 and FePO4. Screening from anions, the valence of the nearest neighbor transition metal, lattice parameters, are identified to be important factors to determine the activation energy for Li diffusion. The calculated activation energy in LiFePO4 and FePO4 are 0.5 eV and 0.27 eV, respectively. It is dependent on the concentration and configuration of lithium ions in the phase boundary regions and a kinetic model for Li extraction and insertion process is proposed.

LiFePO4 thin films have been prepared by pulsed laser deposition method on titanium substrates. The influence of the deposition parameters, e.g. substrate temperature, ambient argon pressure, and post-annealing on the crystallinity and morphology of as-deposited thin films are investigated. Well-crystallized pure olivine-phase is obtained under optimized deposition condition (20–30 Pa, 500 °C). It shows a high electrochemical activity (83% theoretical capacity) at low current density (0.33 μA cm−2, 1/20 C) and elevated testing temperature (45 °C). Moderate post-annealing treatment can enhance the utilization of the films further. The deposition of the film at a too high temperature or post-annealing for too long time could introduce Fe3+ impurities, i.e., Li3Fe2(PO4)3 and Fe4(P2O7)3, which can be easily detected by extending the electrochemical test voltage down to 2.5 V.

Lithium iron phosphate (LiFePO4) thin films were prepared by pulsed laser deposition with an off-axis geometry. Amorphous, needle-like and crystallized granular thin films were prepared on Si and titanium substrates. The preferred orientation of these crystallized LiFePO4 thin films is (120). Microstructures of the deposited films are dependant on the substrate temperature (room temperature, 500 °C and 700 °C) and Ar pressure (5 Pa and 30 Pa) in the chamber. The needle-like film grows following a self-shadowing mechanism. LiFePO4 thin film with high crystallinity shows a voltage plateau which is a typical feature of the phase transition reaction for bulk material, while the sloped profiles are observed clearly in the charging and discharging curves of LiFePO4 thin films with low cystallinity.

Olivine structured LiFe1/4Mn1/4Co1/4Ni1/4PO4 in pure solid solution single phase was obtained using solid-state reaction. For the carbon-coated material, in a voltage range of 3.0–5.1 V (vs. Li/Li+), three voltage plateaus were observed clearly in both galvanostatic discharging and galvanostatic intermittent titration technique (GITT) curves. These three plateaus are corresponding to Co3+/Co2+, Mn3+/Mn2+ and Fe3+/Fe2+ redox couples. No definite plateau can be assigned to Ni3+/Ni2+ redox couple in this voltage range. The overpotentials show the maximum when the redox reaction shifts from one redox couple to another couple. The apparent chemical diffusion coefficients of lithium (D˜Li) are in the same order of magnitude 10−15 cm2 s−1 for all transition reactions.

N doped LiFePO4 has been investigated by using first-principle calculations with the projector augmented wave (PAW) method. The effect of N doping on its crystal structure, charge distribution and transport properties have been studied within the Generalized Gradient Approximation (GGA)+U framework. To maintain charge balance, the valence of the Fe nearest to N appears as Fe3+, and it will benefit for the hopping of electrons. The Elastic band method was used to calculate the activation energy for Li diffusion. N doping leads to slightly lower activation energy.

A series of carbon-coated layered structured Li[CrxLi(1/3−x/3)Ti(2/3−2x/3)]O2 samples (0.3 ≤ x ≤ 0.45) were prepared. Among them, the sample of x = 0.4 shows the highest initial reversible capacity of 207 mAh g−1 at 30 mA g−1 in 2.5–4.4 V. The reversible Li-storage capacities for the samples with high x values (x = 0.4, 0.45) faded slightly while the samples with low Cr content (x = 0.3 and 0.35) showed a capacity increase upon cycling. It was found that the relative intensity ratio of (0 0 3) peak to (1 0 4) peak (R(0 0 3) = I(0 0 3)/I(1 0 4)) is influenced strongly by x value in as-prepared samples. The samples of x = 0.35 and 0.4 turn to a similar structure with low R(0 0 3) value during cycling. These phenomena indicate that the cation mixing of Cr3+ in the lithium layer occurs in as-prepared samples and became more significant upon delithiation and lithiation. This is supposed being a necessary process for Cr-based layered structure materials possessing electrochemical reactivates. The occurrence of the cation mixing is beneficial from the local lattice distortion caused by the short-range ordering between Ti and Li. This is supposed to be helpful for the migration of Cr6+ and Cr3+ at tetrahedral and octahedral sites. Different from the case of LiNiO2, the cation mixing is essential for the transport and storage of lithium in the carbon-coated Li–Cr–Ti–O layered compounds.

The effect of the cation doping on the electronic structure of spinel LiMyMn2−yO4 (M=Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn2O4. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiMyMn2−yO4 can be ascribed to the lower O-2p level introduced by the doping cation M.

Spinel LiMn2O4 was modified by an aqueous fluoride modification method (AFMM). It is found that tiny crystals of potassium manganese trifluoride (KMnF3) grew on the surface of the spinel particles during the electrochemical process. The quantity and the integrality of the KMnF3 crystals were controlled by varying current density and processing time. The mechanisms of the formation of KMnF3 were discussed. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. The chemical diffusion coefficients (Ds) of Li+ were measured by a potential relaxation technique (PRT). The values of Ds obtained in this way are of the order of 10−10–10−9 cm2/s.

LiNi0.5Mn1.5O4−δ which possesses a high voltage of 4.7 V vs. Li+/Li and stable structure has been considered as a promising cathode material for high energy Li-ion batteries. In this study, well-crystallized LiNi0.5Mn1.5O4−δ nanorods with diameter of ∼50 nm and length of ∼500 nm were synthesized by a hydrothermal-solid state process followed by a lithiation reaction. It delivers ∼120 mA h g−1 discharge capacity under 30 mA g−1 at 3.5–4.9 V, and 90 mA h g−1 at 150 mA g−1. An unstable solid electrolyte interphase (SEI) film with thickness 2–10 nm was observed by transmission electron microscopy. The thick and unstable SEI layers are responsible for its low rate performance and low coulombic efficiency during cycling.