The storage of electrical energy at high charge and discharge rate is an important technology in today's society, and can enable hybrid and plug-in hybrid electric vehicles and provide back-up for wind and solar energy. It is typically believed that in electrochemical systems very high power rates can only be achieved with supercapacitors, which trade high power for low energy density as they only store energy by surface adsorption reactions of charged species on an electrode material. Here we show that batteries which obtain high energy density by storing charge in the bulk of a material can also achieve ultrahigh discharge rates, comparable to those of supercapacitors. We realize this in LiFePO(4) (ref. 6), a material with high lithium bulk mobility, by creating a fast ion-conducting surface phase through controlled off-stoichiometry. A rate capability equivalent to full battery discharge in 10-20 s can be achieved.

We present an efficient way to calculate the phase diagram of the quaternary Li-Fe-P-O 2 system using ab initio methods. The ground-state energies of all known compounds in the Li-Fe-P-O 2 system were calculated using the generalized gradient approximation (GGA) approximation to density functional theory (DFT) and the DFT+U extension to it. Considering only the entropy of gaseous phases, the phase diagram was constructed as a function of oxidation conditions, with the oxygen chemical potential, µ O 2 , capturing both temperature and oxygen partial pressure dependence. A modified Ellingham diagram was also developed by incorporating the experimental entropy data of gaseous phases. The phase diagram shows LiFePO 4 to be stable over a wide range of oxidation environments, being the first Fe 2+ -containing phase to appear upon reduction at µ O 2 ) -11.52 eV and the last of the Fe-containing phosphates to be reduced at µ O 2 ) -16.74 eV. Lower µ O 2 represents more reducing conditions, which generally correspond to higher temperatures and/or lower oxygen partial pressures and/or the presence of reducing agents. The predicted phase relations and reduction conditions compare well to experimental findings on stoichiometric and Li-off-stoichiometric LiFePO 4 . For Li-deficient stoichiometries, the formation of iron phosphate phases (Fe 7 (PO 4 ) 6 and Fe 2 P 2 O 7 ) commonly observed under moderately reducing conditions during LiFePO 4 synthesis and the formation of iron phosphides (Fe 2 P) under highly reducing conditions are consistent with the predictions from our phase diagram. Our diagrams also predict the formation of Li 3 PO 4 and iron oxides for Li-excess stoichiometries under all but the most reducing conditions, again in agreement with experimental observations. For stoichiometric LiFePO 4 , the phase diagram gives the correct oxidation products of Li 3 Fe 2 (PO 4 ) 3 and Fe 2 O 3 . The predicted carbothermal reduction temperatures for LiFePO 4 from the Ellingham diagram are also within the range observed in experiments (800-900 °C). The diagrams developed provide a better understanding of phase relations within the Li-Fe-P-O 2 system and serve as a guide for future experimental efforts in materials processing, in particular, for the optimization of synthesis routes for LiFePO 4 .

Phosphate materials are being extensively studied as lithium-ion battery electrodes. In this work, we present a highthroughput ab initio analysis of phosphates as cathode materials. Capacity, voltage, specific energy, energy density, and thermal stability are evaluated computationally on thousands of compounds. The limits in terms of gravimetric and volumetric capacity inherent to the phosphate chemistry are determined. Voltage ranges for all redox couples in phosphates are provided, and the structural factors influencing the voltages are analyzed. We reinvestigate whether phosphate materials are inherently safe and find that, for the same oxidation state, oxygen release happens thermodynamically at lower temperature for phosphates than for oxides. These findings are used to recommend specific chemistries within the phosphate class and to show the intrinsic limits of certain materials of current interest (e.g., LiCoPO 4 and LiNiPO 4 ).

We present an analysis of the thermal reduction of delithiated LiMnPO 4 and LiFePO 4 based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent experimental findings that MnPO 4 decomposes at a much lower temperature than FePO 4 , thereby potentially posing larger safety issues for LiMnPO 4 cathodes. We find that while substantial oxygen is released as MnPO 4 reduces to Mn 2 P 2 O 7 , the mixed valence phases that form in the decomposition process of FePO 4 limit the amount of oxygen evolved.

Chemical reactions at the solid electrolyte (SE) and Li metal interface form an interphase before electrochemical reactions occur. This study investigates the effects of the chemically formed interphase between Li metal and Li1.5Al0.5Ge1.5(PO4)3 (LAGP) on cell failures under various experimental conditions. LAGP forms a black interphase by chemically reacting with Li metal. The interphase comprises a stoichiometrically changed LAGP and Li-related oxides and behaves as a mixed ionic and electronic conductor with the electronic conductivity dominating. Thus, upon application of an electrical current to Li metal anode, most of the Li ions can be reduced at the SE side surface of the interphase rather than the Li metal side, causing a local volumetric increase that triggers cracks in the SE. This crack formation process continues the pulverization of SE, leading to a gradual increase in cell resistance. Under cell operating conditions, electrochemical reactions with the chemically formed interphase can lead to the mechanical deterioration of the SE, leading to cell failure. Furthermore, the chemically formed interphase between melted Li and LAGP above 200 °C induces a rigorous chemical reaction with Li that leads to a thermal runaway. The chemical stability of the SE against Li metal can strongly affect the solid-state cell’s electrical properties, mechanical integrity, and thermal stability.

The Na superionic conductor (aka Nasicon, NaZrSiPO, where 0 ≤ x ≤ 3) is one of the promising solid electrolyte materials used in advanced molten Na-based secondary batteries that typically operate at high temperature (over ∼270 °C). Nasicon provides a 3D diffusion network allowing the transport of the active Na-ion species (i.e., ionic conductor) while blocking the conduction of electrons (i.e., electronic insulator) between the anode and cathode compartments of cells. In this work, the standard Nasicon (NaZrSiPO, bare sample) and 10 at% Na-excess Nasicon (NaZrSiPO, Na-excess sample) solid electrolytes were synthesized using a solid-state sintering technique to elucidate the Na diffusion mechanism (i.e., grain diffusion or grain boundary diffusion) and the impacts of adding excess Na at relatively low and high temperatures. The structural, thermal, and ionic transport characterizations were conducted using various experimental tools including X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). In addition, an ab initio atomistic modeling study was carried out to computationally examine the detailed microstructures of Nasicon materials, as well as to support the experimental observations. Through this combination work comprising experimental and computational investigations, we show that the predominant mechanisms of Na-ion transport in the Nasicon structure are the grain boundary and the grain diffusion at low and high temperatures, respectively. Also, it was found that adding 10 at% excess Na could give rise to a substantial increase in the total conductivity (e.g., ∼1.2 × 10 S/cm at 300 °C) of Nasicon electrolytes resulting from the enlargement of the bottleneck areas in the Na diffusion channels of polycrystalline grains.

Ordered LiNi 0.5 Mn 1.5 O 4 was synthesized through a solid-state reaction. Even though the material has a particle size of 3-5 m, it shows very high rate capability and excellent capacity retention. The capacity is as high as Ϸ78 mAh/g at a 167C discharge rate. This high discharge rate performance is consistent with first-principles calculations of the activation barrier for lithium motion, which predict the lithium diffusivity in this material to be around 10 −9-10 −8 cm 2 /s. We also systematically investigated the effect of several cell components and electrode construction on the measured rate performance and conclude that care has to be taken to remove all other rate limitations from the cell to measure the rate performance of an electrode material.

We investigated the structural stability and electrochemical properties of LiMnBO 3 in the hexagonal and monoclinic form with ab initio computations and, for the first time, report electrochemical data on monoclinic LiMnBO 3 . In contrast to the negligible Li-storage capacity in the hexagonal LiMnBO 3 , a second cycle discharge capacity of 100 mAh/g was achieved in the monoclinic LiMnBO 3 , with good capacity retention over multiple cycles. Elevated temperature cycling indicates that the capacity of monoclinic LiMnBO 3 is kinetically limited, and further improvement may be expected by addressing the Li ion and/or electron transport limitations.