3227wileyonlinelibrary.com be adapted almost directly to the sodium system. Sodium equivalents of lithiumcontaining electrode materials, such as oxides, sulfi des, phosphates, pyrophosphates, fl uorophosphates, and alloying metals, have been evaluated as electrode materials for SIBs. [1][2][3][4][5][6] Among the positive electrode materials for SIBs, layered oxides (NaTMO 2 , TM = transition metals) are most attractive due to their large capacity, simple synthesis, and structural stability. [7][8][9][10][11][12][13][14] Various transition metal elements can be substituted into the layered structure, similar to layered lithium compounds, and this will infl uence the structural stability of sodium-ion removal, operating voltage, capacity, and cyclability. In contrast to lithium systems, different stacking structures for Na layered oxides can be examined because of the preferred prismatic coordination of the larger sodium ions. Positive electrode materials with a P2 layered structure by Delmas' notation [ 15 ] have better cyclability and structural stability during electrochemical reactions than those with an O3 layered structure. [ 16 ] The trigonal prismatic site is more favorable for the diffusion of sodium ions, [17][18][19][20] and the diffusion kinetics for Na + can thus be more effi cient in the P2 layered structure compared to those in the O3 layered structure, which is common in lithium intercalation compounds. P2 layered materials with various transition metal compositions for SIBs have been studied, such as Mn-Co, Ni-Mn, and Fe-Mn. [ 9,17,[21][22][23][24][25] Moreover, several approaches for the doping of two-component P2 layered materials with transition metals have also been introduced, [ 8 ] including Co doping on Ni-Mn compounds, [ 26,27 ] Ni doping on Co-Mn systems, [ 28,29 ] and Ni doping on Fe-Mn system. [ 30 ] However, few studies on the Fe-Mn-Co system with P2 stacking have been reported compared to Ni-Fe-Mn or Ni-Co-Mn P2 materials. [ 31,32 ] Co can facilitate the oxidation of Fe atoms, as reported in Li compounds, [ 33 ] and Co can likely stabilize the oxidized state in the layered structure, especially for Fe-containing layered materials. [ 34 ] In addition, Wang et al. reported that Co suppresses the irreversibility of P2-Na 2/3 Mn y Co 1− y O 2 materials. [ 23 ] A similar behavior in the P2-Na-Fe-Co-Mn oxides would be expected.In this paper, P2-Na 0.7 [(Fe 0.5 Mn 0.5 ) 1− x Co x ]O 2 ( x = 0, 0.05, 0.10, and 0.20) was synthesized by a solid-state reaction, and the electrochemical performance of the P2-Fe-Mn-Co system Sodium layered oxides with mixed transition metals have received signifi cant attention as positive electrode candidates for sodium-ion batteries because of their high reversible capacity. The phase transformations of layered compounds during electrochemical reactions are a pivotal feature for understanding the relationship between layered structures and electrochemical properties. A combination of in situ diffraction and ex situ X-ray absorption spectroscopy reveals the phase t...

Carbon black (CB) additives commonly used to increase the electrical conductivity of electrodes in Li-ion batteries are generally believed to be electrochemically inert additives in cathodes. Decomposition of electrolyte in the surface region of CB in Li-ion cells at high voltages up to 4.9 V is here studied using electrochemical measurements as well as structural and surface characterizations. LiPF 6 and LiClO 4 dissolved in ethylene carbonate:diethylene carbonate (1:1) were used as the electrolyte to study irreversible charge capacity of CB cathodes when cycled between 4.9 V and 2.5 V. Synchrotron-based soft X-ray photoelectron spectroscopy (SOXPES) results revealed spontaneous partial decomposition of the electrolytes on the CB electrode, without applying external current or voltage. Depth profile analysis of the electrolyte/cathode interphase indicated that the concentration of decomposed species is highest at the outermost surface of the CB. It is concluded that carboxylate and carbonate bonds (originating from solvent decomposition) and LiF (when LiPF 6 was used) take part in the formation of the decomposed species. Electrochemical impedance spectroscopy measurements and transmission electron microscopy results, however, did not show formation of a dense surface layer on CB particles. The growth of earth's population with concomitant increase in energy consumption require development of renewable energy conversion technologies coupled with advanced energy storage systems like lithium batteries.1,2 In order to increase the power density in Li-ion batteries, much research is focused on developing cathode materials that can operate at high voltages (above 4.5 V vs. Li/Li + ) with a high capacity, high cycling stability, and good rate capability.3-5 However, at high voltages, all the components of positive electrodes including the Al current collector, polymer binders, conductive additives, and other possible additives have an increased risk of degradation. In addition, one of the main issues with high voltage batteries is the instability of common aprotic electrolytes at voltage above 4.5 V. 6,7 The stability of the electrolyte/cathode interphase is related to the chemistry of electrolyte solvents and salts and also to the chemistry of the components of the cathode.Carbon black (CB) additives are one of the main constituents of cathodes, added to increase the electrical percolation and thus the electronic conductivity. 8,9 Though the weight percentage of CB in commercial batteries is generally very small, it composes a rather large part of the internal surface area of a cathode due to its small particle size (≈50 nm), low density, and high surface area. CBs are generally thought of being an electrochemically inert additive in cathodes, but few studies have investigated the role of CBs at high voltages and have indicated that CBs exhibit irreversible electrochemical reactions resulting in appreciable irreversible charge capacities. [10][11][12][13][14][15][16][17][18] This charge capacity is attributed to oxid...

The charge and discharge performance of an all-solid-state lithium battery with the LiBH 4 -LiI solid solution as an electrolyte is reported. Lithium titanate (Li 4 Ti 5 O 12 ) was used as the positive electrode and lithium metal as the negative electrode. The performance of the all-solid-state cell is compared with a cell with an identical electrode setup but a liquid electrolyte (1 M LiPF 6 in EC:DMC). All measurements were carried out at a temperature of 60 • C. For the all-solid-state cells, 81% of the theoretical discharge capacity is reached for a discharge rate of 10 μA, but a capacity fade of 1.6% per charge-discharge cycle is observed. The electrochemical stability of the LiBH 4 -LiI solid solution was investigated using cyclic voltammetry and is found to be limited to 3 V. The impedance of the battery cells was measured using impedance spectroscopy. A strong correlation is found between the change in the discharge capacity of the cells and changes in the cell impedance over 200 charge-discharge cycles. This is expectedly due to the possible formation of passivating areas in the cell and/or loss of contact area between the electrolyte and the electrodes.

In commercial Fe-based batteries the Fe2+/Fe3+ oxidation states are used, however by also utilizing the Fe4+ oxidation state, intercalation of up to two Li ions per Fe ion could be possible. In this study, we investigate whether Fe4+ can be formed and stabilized in β-Li3Fe2(PO4)3. The work includes in situ synchrotron X-ray powder diffraction studies (XRPD) during charging of β-Li3Fe2(PO4)3 up to 5.0 V vs. Li/Li+. A novel capillary-based micro battery cell for in situ XRPD has been designed for this. During charge, a plateau at 4.5 V was found and a small contraction in volume was observed, indicating some Li ion extraction. The volume change of the rhombohedral unit cell is anisotropic, with a decrease in the a parameter and an increase in the c parameter during the Li ion extraction. Unfortunately, no increased discharge capacity was observed and Mössbauer spectroscopy showed no evidence of Fe4+ formation. Oxidation of the organic electrolyte is inevitable at 4.5 V but this alone cannot explain the volume change. Instead, a reversible oxygen redox process (O2− → O−) could possibly explain and charge compensate for the reversible extraction of lithium ions from β-Li3Fe2(PO4)3.

Possible changes in the oxidation state of the oxygen ion in the lithium iron phosphate Li3Fe2(PO4)3 at high voltages in lithium‐ion (Li‐ion) batteries are studied using experimental and computational analysis. Results obtained from synchrotron‐based hard X‐ray photoelectron spectroscopy and density functional theory (DFT) show that the oxidation state of O2− ions is altered to higher oxidation states (Oδ−, δ<2) upon charging Li3Fe2(PO4)3 to 4.7 V.