The first charge–discharge cycling behaviors of two sets of Li–Ni–Mn–Co type positive electrode materials were compared. The samples in each set have similar Ni–Mn–Co ratios but different Li-to-total metal ratio (Li/M). The samples that were Li-rich with a Li­[Li x M(1–x)]­O2 structure showed a typical 4.5 V “oxygen loss” plateau and a typical irreversible capacity loss near 25%. Surprisingly, other samples with lower Li/M ratios that still exhibited a 4.5 V “oxygen loss” plateau exhibited an irreversible capacity loss as low as 4.0% of their first charge capacity. XRD analysis revealed that all samples were single-phase layered oxides. A separate and a detailed XRD analysis combined with dQ/dV analysis showed that the reduced irreversible capacity loss was not caused by the admixture of a spinel phase. ICP-OES results and the oxidation state versus atomic occupancy rules suggested the presence of metal site vacancies in the pristine materials with low IRC, which were confirmed by densities measured with a helium pycnometer. The results presented here show that the small irreversible capacity is a consequence of (a) metal site vacancies, leading to Li­[□ q M(1–q)]­O2 structures, where □ is a metal site vacancy, which leads to (b) no Li atoms in the transition metal layer. These materials still have Li/M > 1, so they are “Li-rich”, but they are “traditional layered materials” with no Li in the transition metal layer. This study identifies a new route for fabricating high capacity Li-rich positive electrode materials with small irreversible capacity loss.

Li [Ni 0.6 Mn 0.2 Co 0.2 ]O 2 (NMC622) is one of the most attractive positive electrode materials for Li-ion cells. Ultra high precision coulometry, impedance spectroscopy, gas evolution and long-term cycling were used to explore the combined impact of several electrolyte additives and an Al 2 O 3 surface coating on NMC622/graphite cells tested to 4.4 V at 40 • C. An excellent correlation between the short-term coulombic efficiency measurements and the long-term cycling results was noted. An electrolyte containing 1 M LiPF 6 in ethylene carbonate: ethyl methyl carbonate (3:7 by weight) with additives 2 wt% prop-1-ene-1,3 sultone (PES) + 1 wt% methylene methane disulfonate (MMDS) + 1 wt% tris (trimethylsilyl) phosphite (TTSPi) (this electrolyte is called PES 211) was found to give the best performance in both coated and uncoated NMC622/graphite cells. The surface coating was found to improve coulombic efficiency, reduce impedance and improve capacity retention in all cases studied. However, uncoated NMC622/graphite cells with PES211 were found to perform better than coated NMC622/graphite cells which used any other electrolyte. The special synergy between Al 2 O 3 -coated NMC622 and PES211 needs to be understood so that even better coatings and electrolyte combinations can be developed. Li-ion batteries are the best power sources for applications ranging from portable electronics to electric vehicles.1,2 Properties such as high energy density, high power, low cost and longer life-time (calendar life) are often desired for such applications. Increasing the upper cutoff voltage in Li[Ni 1-x-y Mn y Co z ]O 2 "NMC" Li-ion cells results in increased energy density.3 However, increasing the upper cutoff voltage increases undesirable side reactions 4 (parasitic) at the electrodeelectrolyte interface and shortens the lifetime of the cell.5 While high energy density is important for many applications, lifetime is also important.Electrochemical oxidation of electrolytes occurring at the positive electrode at high voltage is a primary cause of cell failure. Hence methods to hinder these parasitic reactions should be developed. Coating the surface of the positive electrode and using electrolyte additives are two popular ways to suppress the parasitic reactions and thus improve the cell lifetime. As an example of work of the Umicore authors of this paper, Figure 1 shows a comparison between the charge-discharge capacity versus cycle number of Al 2 O 3 -coated NMC622/graphite and uncoated NMC622/graphite pouch type cells built at Umicore. The cells in Figure 1 used the same electrolyte which was supplied by PanaxEtec. They were tested at 1 C charge with a constant voltage hold till C/20 followed by a 1 C discharge between 3.0 V and 4.35 V at 25• C. The cells had a capacity of about 700 mAh with an initial typical gravimetric energy density of 180 Wh/kg @ 4.35 V. Figure 1 shows that: * Electrochemical Society Student Member.* * Electrochemical Society Fellow. z E-mail: jeff.dahn@dal.ca 1) Uncoated NMC622 -compared to Al 2 O 3 ...

Li-rich positive electrode materials (e.g. Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 ) are potential candidates for high energy density Li-ion batteries. [1][2][3] They are capable of delivering reversible specific capacities up to 250 mAh/g 4,5 at an average discharge potential of ∼3.5 V vs Li metal. 6Understanding the structure of Li-rich materials is essential to improve their properties and performance. Li-rich materials are layered transition metal (TM) oxides comprised of alternating layers of metal atoms (Li or TM) and oxygen atoms. 7 Compared to non-Li-rich layered transition metal oxides such as LiCoO 2 , the Li/TM ratio is greater than 1 for Li-rich layered transition metal oxides and usually Li atoms occupy the TM layer in addition to the Li layer.8 Li 2 MnO 3 is a typical example of such a material in which 1 4 of the Li atoms occupy sites in the TM layer.Li-rich layered transition metal oxides can be defined as O3 structures with A-B-C-A-B-C stacking but the arrangement of the atoms in the TM layer is different from that in other layered materials. 9 The presence of Li + ions with large ionic radii (0.74 Å) and small sized Mn 4+ ions (0.54 Å) in the TM layer causes an in-plane ordering resulting in a √ 3a × √ 3a superstructure or superlattice 10 and changes the symmetry from R-3m to C2/m. The superlattice ordering in the TM layer results in superstructure Bragg peaks in the range of ∼ 20• to 35

The effects of three esters incorporated as co-solvents in 1.2 M LiPF 6 EC:EMC:DMC (25:5:70 by volume %) electrolyte were studied in Li[Ni 1-x-y Co x Al y ]O 2 /Graphite-SiO pouch cells. The esters: methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB) were compared in a variety of tests on the cells. Storage tests at 60 • C at both 4.2 V and 2.5 V demonstrated that MB and MP outperformed EA and cells containing 20% MP showed the least voltage drop during the 500 h storage period. In long-term cycling tests at 40 • C, cells containing up to 20% ester exhibited similar capacity retention compared to cells with only EC:EMC:DMC solvent. Unwanted lithium plating could be suppressed in cells with 20% MP during charging above 2C compared to ester-free cells, which showed the onset of unwanted lithium plating at 1.8C. This improvement is due to the increased Li + conductivity of electrolytes containing MP. In addition, the use of up to 40% MP did not enhance reactivity between the charged electrode materials and electrolyte at elevated temperatures according to accelerating rate calorimetry measurements. To further satisfy the development of electric vehicles (EVs), Liion batteries need to be designed with higher volumetric energy density, longer cycling life, higher rate capability, lower cost and so forth. These improvements can be fully or partially achieved by applying new positive and/or negative electrode materials, suitable electrolytes and additives. For instance, Li[Ni 0.85 Co 0.10 Al 0.05 ]O 2 (NCA), as a typical positive electrode material, exhibits high energy density and capacity, and has been used in many EVs 1,2 and power tools. Esters are beneficial co-solvents that can improve the physical properties of carbonate-based electrolytes due to their low freezing points, high ionic conductivity of the resulting electrolytes and low viscosity. Esters have been mainly studied for the low temperature application of Li-ion cells, and compounds of interest include, ethyl acetate (EA), methyl butyrate (MB), ethyl butyrate (EB), methyl propionate (MP), ethyl propionate (EP), etc.4-12 Of these ester solvents, it has been reported that those with low molecular weight (e.g. EA) performed well initially but degraded during longterm cycling due to high reactivity with the negative electrode. 4,5 The most studied positive electrode materials in combination with esters in Li-ion batteries were LiCoO 2 5-8 and LiNi x Co 1-x O 2 , and the latter was mainly studied by Smart et al. 4,9 In our group, EA and MP used as a sole electrolyte solvent have been studied in Li [Ni 0.33 Large fractions of esters are now appearing in commercial lithium-ion cells. For example we have performed analysis on some LiCoO 2 /graphite Li-ion cells, rated for 4.35 V, and found an electrolyte that was LiPF 6 dissolved in MP:FEC:PC in ratios of 56:37:7 by weight. Presumably this novel electrolyte system was selected to improve high voltage performance and improve rate capability. It is important to determine if MP would be the best...

A comprehensive search for Li-ion battery positive electrode materials that can simultaneously exhibit low irreversible capacity loss (IRC) (∼10% or less) and high reversible capacity (>240 mAh/g) was performed in the Li−Ni−Mn−Co−O pseudoquaternary system. An array of high-capacity Li-rich layered oxides, most of which show an "oxygen release" plateau during the first charge, were synthesized with a wide range of Ni, Mn, and Co compositions, and their first-cycle electrochemical properties were investigated. Low-IRC materials could be synthesized at many Ni− Mn−Co combinations by synthesizing with an amount of lithium lower than that required by site occupation and oxidation state rules. Many of these "Li-deficient" low-IRC materials were found to be single-phase layered materials with inherent metal-site vacancies in their pristine state. For such single-phase materials, the amount of IRC depends on the concentration of metal-site vacancies in their pristine state. Increasing the Li deficiency eventually caused the appearance of the spinel phase, which, when it appears, lowers the IRC, irrespective of the Ni−Mn−Co precursor composition. The number of metal-site vacancies that can be incorporated into the single-phase layered materials depends on the overall metal composition, especially the Co concentration. Low-IRC behavior is correlated to the fraction of metal-site vacancies in the layered phase in both the single-phase and the twophase materials. 7 Li nuclear magnetic resonance (NMR) studies of low-IRC materials revealed the relative population of Li between the Li and TM layer. Formula unit calculation based on 7 Li NMR results suggests that metal-site vacancies preferably occupy the sites in the Li layer, which could provide room for the intercalation of extra Li into the structure, hence reducing the irreversible capacity.