Finding a viable electrolyte for next-generation 5 V-class lithium-ion batteries is of primary importance. A long-standing obstacle has been metal-ion dissolution at high voltages. The LiPF6 salt in conventional electrolytes is chemically unstable, which accelerates transition metal dissolution of the electrode material, yet beneficially suppresses oxidative dissolution of the aluminium current collector; replacing LiPF6 with more stable lithium salts may diminish transition metal dissolution but unfortunately encounters severe aluminium oxidation. Here we report an electrolyte design that can solve this dilemma. By mixing a stable lithium salt LiN(SO2F)2 with dimethyl carbonate solvent at extremely high concentrations, we obtain an unusual liquid showing a three-dimensional network of anions and solvent molecules that coordinate strongly to Li(+) ions. This simple formulation of superconcentrated LiN(SO2F)2/dimethyl carbonate electrolyte inhibits the dissolution of both aluminium and transition metal at around 5 V, and realizes a high-voltage LiNi0.5Mn1.5O4/graphite battery that exhibits excellent cycling durability, high rate capability and enhanced safety.

Superconcentrated Electrolytes for a High-Voltage Lithium-Ion Battery

There is an urgent need for low-cost, resource-friendly, high-energy-density cathode materials for lithium-ion batteries to satisfy the rapidly increasing need for electrical energy storage. To replace the nickel and cobalt, which are limited resources and are associated with safety problems, in current lithium-ion batteries, high-capacity cathodes based on manganese would be particularly desirable owing to the low cost and high abundance of the metal, and the intrinsic stability of the Mn4+ oxidation state. Here we present a strategy of combining high-valent cations and the partial substitution of fluorine for oxygen in a disordered-rocksalt structure to incorporate the reversible Mn2+/Mn4+ double redox couple into lithium-excess cathode materials. The lithium-rich cathodes thus produced have high capacity and energy density. The use of the Mn2+/Mn4+ redox reduces oxygen redox activity, thereby stabilizing the materials, and opens up new opportunities for the design of high-performance manganese-rich cathodes for advanced lithium-ion batteries.

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Reversible Mn 2+ /Mn 4+ double redox in lithium-excess cathode materials

The accelerating development of technologies requires a significant energy consumption, and consequently the demand for advanced energy storage devices is increasing at a high rate. In the last two decades, lithium-ion batteries have been the most robust technology, supplying high energy and power density. Improving cathode materials is one of the ways to satisfy the need for even better batteries. Therefore developing new types of positive electrode materials by increasing cell voltage and capacity with stability is the best way towards the next-generation Li rechargeable batteries. To achieve this goal, understanding the principles of the materials and recognizing the problems confronting the state-of-the-art cathode materials are essential prerequisites. This Review presents various high-energy cathode materials which can be used to build next-generation lithium-ion batteries. It includes nickel and lithium-rich layered oxide materials, high voltage spinel oxides, polyanion, cation disordered rock-salt oxides and conversion materials. Particular emphasis is given to the general reaction and degradation mechanisms during the operation as well as the main challenges and strategies to overcome the drawbacks of these materials.

Advances in the Cathode Materials for Lithium Rechargeable Batteries.

For lithium-ion rechargeable batteries to meet society's ever-growing demands in electrical energy storage, e.g. for the electrification of transportation, for portable electronics and for grid storage applications, novel electrode materials with a large charge storage capacity and a high energy density are needed. Over the last five years, several experimental and theoretical studies have demonstrated the feasibility of disordered rocksalt (DRX) cathodes, that is, lithium transition metal oxide cathodes with a crystalline rocksalt structure but with a disordered arrangement of lithium and transition metal on the cation lattice. We provide here an overview of the current understanding of DRX materials, in terms of their structural and compositional characteristics, as well as their electrochemical properties. We also present important considerations for the design of high performance DRX cathodes and suggest future research directions. Because no specific order is needed, DRX compounds can be composed of a wide variety of transition metal species, which can create long-term benefits for the lithium battery industry by making it less reliant on scarce and expensive raw materials. While some DRX compositions can simply be synthesized at high temperature to induce thermal cation disorder, other compositions require mechanochemical methods to induce a disordered arrangement of cation species. Cation disorder leads to unique lithium transport properties, small volume changes during charge–discharge cycling and sloping electrochemical profiles. Fluorine substitution for oxygen and the incorporation of high-valent d0 transition metals in the bulk DRX structure are two strategies used to increase the lithium content in the material, improve lithium percolation and to keep the valence of redox-active metal species low so that high transition metal redox capacity can be obtained. Short-range cation order, which is affected by thermal treatment, metal composition and fluorine substitution, has a significant impact on electrochemical performance. Moreover, fluorine substitution for oxygen improves long-term capacity retention by significantly reducing anion-based charge compensation mechanisms during charge. Fluorinated DRXs have recently demonstrated reversible capacities >300 mA h g−1 and extremely high energy densities approaching 1000 W h kg−1, holding promise for a nearly two-fold increase in the energy density of commercial lithium-ion batteries.

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Cation-disordered rocksalt transition metal oxides and oxyfluorides for high energy lithium-ion cathodes

A subject for the invention is to improve the cycle characteristics of a high-capacity secondary battery containing an active material packed at a high density, by using a particulate active material having a low aspect ratio. The invention relates to a nonaqueous-electrolyte secondary battery comprising a negative electrode and a positive electrode each capable of occluding/releasing lithium, a separator, and a nonaqueous electrolyte solution comprising a nonaqueous solvent and a lithium salt, characterized in that the separator comprises a porous film made of a thermoplastic resin containing an inorganic filler, and at least either of the following is satisfied: the active material contained in the negative electrode is a particulate active material having an aspect ratio of from 1.02 to 3; and the active material contained in the positive electrode is a particulate active material having an aspect ratio of from 1.02 to 2.2.

Nonaqueous electrolyte secondary battery

Reversible Li storage for nanosize cation/anion-disordered rocksalt-type oxyfluorides: LiMoO 2 - x LiF (0 ≤ x ≤ 2) binary system

A titanium-substituted lithium-excess molybdenum oxyfluoride, Li2.1–yTi0.2Mo0.7O2F, is synthesized by mechanical milling and tested as a positive electrode material in a conventional carbonate-based electrolyte or concentrated electrolyte. Reversibility as the electrode material is significantly improved by suppression of dissolution of the molybdenum oxyfluoride on electrochemical cycles. Li2.1–yTi0.2Mo0.7O2F delivers a reversible capacity of 265 mA h g–1 after 30 cycles at a rate of 50 mA g–1 with concentrated electrolyte. This finding contributes to the development of high-energy and long-cycle-life rechargeable lithium batteries with lithium-excess metal oxyfluorides in the future.

Improved Electrode Performance of Lithium-Excess Molybdenum Oxyfluoride: Titanium Substitution with Concentrated Electrolyte

A need exists in the prior art for a battery having a high capacity retention rate. The present invention provides a positive electrode active material comprising: a compound that is represented by the compositional formula (1) indicated below and that has a crystal structure belonging to the space group FM3-M; and a lithium ion derivative. Formula (1): Li x Me y O α F β . In formula (1), Me is one or more elements selected from the group consisting of Mn, Co, Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti, W, Ge, Mo, Sn, Bi, Cu, Mg, Ca, Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V, and Cr, and the conditions 1.7 ≤ x ≤ 2.2, 0.8 ≤ y ≤ 1.3, 1 ≤ α ≤ 2.5, and 0.5 ≤ β ≤ 2 are satisfied.

Positive electrode active material and battery

A cathode active material contains a compound having a crystal structure of space group FM-3M and represented by composition formula (1): LixMeyO2 . . . (1). In the formula, Me represents any of the following: Mn; Mn and one or two or more elements selected from the group consisting of Co, Fe, Sn, Cu, Mo, Bi, V, and Cr; Ni, Mn, and one or two or more elements selected from the group consisting of Co, Fe, Sn, Cu, Mo, Bi, V, and Cr; and one or two or more elements selected from the group consisting of Ni, Co, Fe, Sn, Cu, Mo, Bi, V, and Cr. In addition to this, the following conditions are met: 0.5≦x/y≦3.0; and 1.5≦x+y≦2.3.

Cathode active material and battery

An object of the present invention or a problem to be solved by the present invention is to provide, as a material for use as a positive electrode of a sodium secondary battery, a novel material that allows the resulting battery to have capacity characteristics superior to those of conventional batteries. The composite metal oxide of the present invention has a composition represented by the general formula Na x Me y O 2 , where Me is at least one selected from the group consisting of Fe, Mn, and Ni, x satisfies 0.8<x≦1.0, and y satisfies 0.95≦y<1.05, and consists of a P2 structure. The sodium secondary battery of the present invention includes: a positive electrode ( 13 ) containing the composite metal oxide of the present invention; a negative electrode ( 16 ) containing a material capable of absorbing and desorbing Na ions; and an electrolyte containing Na ions and anions.

Ryuichi Natsui

Papers

Reversible Li storage for nanosize cation/anion-disordered rocksalt-type oxyfluorides: LiMoO 2 - x LiF (0 ≤ x ≤ 2) binary system

Improved Electrode Performance of Lithium-Excess Molybdenum Oxyfluoride: Titanium Substitution with Concentrated Electrolyte

Positive electrode active material and battery

Cathode active material and battery

Composite metal oxide, method for producing composite metal oxide, and sodium secondary battery