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The Li-ion Rechargeable Battery: A Perspective
John B. Goodenough, and Kyu-Sung Park
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Jan 2013
Downloaded from pubs.acs on January 13, 2013
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The Li-ion Rechargeable Battery: A Perspective
John B. Goodenough *
and Kyu-Sung Park
Texas Materials Institute and Materials Science and Engineering Program, The University of
Texas at Austin, Austin, Texas 78712, United States
E-mail: jgoodenough@mail.utexas.edu
Abstract Each cell of a battery stores electrical energy as chemical energy in two electrodes, a reductant (anode) and an oxidant (cathode), parated by an electrolyte that transfers the ionic component of the chemical reaction inside the cell and forces the electronic component outside the battery. The output on discharge is an external electronic current I at a voltage V for a time ∆t . The chemical reaction of a rechargeable battery must be reversible on the application of a charging I and V . Critical parameters of a rechargeable battery are safety, the density of energy that can be stored at a specific power input and retrieved at a specific power output, the cycle and shelf life, the storage efficiency, and the cost of fabrication.
Conventional ambient-temperature rechargeable batteries have solid electrodes and a liquid electrolyte. The positive electrode (cathode) consists of a host framework into which the mobile (working) cation is inrted reversibly over a finite solid-solution range. The solid-solution range, which is reduced at higher current by the rate of transfer of the working ion across electrode/electrolyte interfaces and within a host, limits the amount of charge per electrode formula unit that can be transferred over the time ∆t = ∆t(I). Moreover, the difference between the energies of the LUMO and the HOMO of the electrolyte, i.e. the electrolyte window, determines the maximum voltage for a long shelf and cycle life. The maximum stable voltage with an aqueous electrolyte is 1.5
V; the Li-ion rechargeable battery us an organic electrolyte with a larger window, which increa the density of stored energy for a given ∆t. Anode or cathode electrochemical potentials outside the electrolyte window can increa V , but they require formation of a passivating surface layer that must be permeable to Li + and capable of adapting rapidly to the changing electrode surface area as the electrode changes volume during cycling. A passivating surface layer adds to the impedance of the Li + transfer across the
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electrode/electrolyte interface and lowers the cycle life of a battery cell. Moreover, formation of a passivation layer on the anode robs Li from the cathode irreversibly on an initial charge, further lowering the reversible ∆t. The problems plus the cost of quality control of manufacturing plague development of Li-ion rechargeable batteries that can compete with the internal combustion engine for powering electric cars and that can provide the needed low-cost storage of electrical energy generated by renewable wind and/or solar energy.
Chemists are contributing to incremental improvements of the conventional strategy by (1) investigati
张铭轩ng and controlling electrode passivation layers, (2) improving the rate of Li+ transfer across electrode/electrolyte interfaces, (3) identifying electrolytes with larger windows while retaining a Li+ conductivity σLi > 10-3 S cm-1, (4) synthesizing electrode morphologies that reduce the size of the active particles while pinning them on current collectors of large surface area accessible by the electrolyte, (5) lowering the cost of cell fabrication, (6) designing displacement-reaction anodes of higher capacity that allow a safe, fast charge, (7) designing alternative cathode hosts. However, new strategies are needed for batteries that go beyond powering hand-held devices. The strategies include (1) the u of electrode hosts with two-electron redox centers, (2) replacing the cathode hosts (a) by materials undergoing displacement reactions, e.g. sulfur, (b) by liquid cathodes that may contain flow-through redox molecules, (c) by catalysts for air cathodes, and /or (3) by the development of a Li+ solid electrolyte parator membrane that allows an organic and aqueous liquid electrolyte on the anode and cathode sides, respectively. Opportunities exist for the chemist to bring together oxide and polymer or graphene chemistry in imaginative morphologies.
Introduction
小黄人图片Modern civilization has become dependent on fossil fuels of finite supply and uneven global distribution, which has two problematic conquences: (1) vulnerability of nation states to fossil-fuel
imports and (2) CO2 emissions that are acidifying our oceans and creating global warming. The Li-ion rechargeable battery (LIB) has enabled the wireless revolution of cell phones, lap-top computers, digital cameras, and i-pads that has transformed global communication. This technology has raid the following pressing question: Can this or another electrochemical technology enable modern civilization to cure a sustainable, distributed energy Page 2 of 29
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supply for all people and reduce the imprint on air-pollution of the internal combustion engine and coal-fired power plants? A portable rechargeable battery and the electrochemical capacitor can, together, displace the internal combustion engine by powering electric vehicles; but how safely, at what cost, and over how great a driving range? A stationary rechargeable battery can store efficiently electrical energy generated by solar and/or wind power, and it can provide a distributed or a centralized energy store; but for how long a shelf and cycle life, with how rapid a respon to a power outage or fluctuation in the grid, and with how large a capacity at a competitive cost?
A battery is made of one or more interconnected electrochemical cells each giving a current at a voltage for a time ∆t . The output current I and/or time ∆t to depletion of the stored energy in a battery can be incread by enlarging the area of the electrodes or connecting cells in parallel; the voltage V for a desired power P = IV by connecting cells in ries. Here we address only issues related to strategies for individual rechargeable battery cells; the management of the individual cells of a battery becomes more complex, as does the cost, the larger the number of cells needed for a given battery application.
Electrochemical Cells
An electrochemical cell consists of two electrodes, the anode and the cathode , parated
by an electrolyte . The electrolyte may be a liquid or a solid. Solid electrolytes are ud with gaous or liquid electrodes; they may be ud with solid electrodes, but solid-solid interfaces are problematic unless the solid electrolyte is a polymer or the solid electrodes are thin. Solid electrodes parated by a liquid electrolyte are kept apart by an electrolyte-permeable parator .
The electrolyte conducts the ionic component of the chemical reaction between the anode and the cathode, but it forces the electronic component to traver an external circuit where it does work, Fig.
1. Becau the ionic mobility in the electrolyte is much smaller than the electronic conductivity in a metal, a cell has large-area electrodes parated by a thin electrolyte; metallic current collectors deliver electronic current from/to the redox centers of the electrodes to/from posts that connect to the external circuit. A rechargeable cell has a reversible chemical reaction at the two electrodes.
During discharge and charge , an internal battery resistance R b to the ionic current I i = I reduces the output voltage V dis from the open-circuit voltage V oc by a polarization
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η = I dis R b and increas the voltage V ch required to rever the chemical reaction on charge by an overvoltage η = I ch R b :
V dis = V oc – η(q,I dis ) (1.1) V ch = V oc + η(q,I ch )
(1.2)
where q reprents the state of charge. The percent efficiency of a cell to store energy at a fixed current I is
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where Q is the total charge per unit weight (Ah kg -1) or per volume (Ah L -1) transferred by the current I = dq/dt on discharge or charge. Q(I) is referred to as the cell capacity for a given I ; Q depends on I becau the rate of transfer of ions across electrode/electrolyte interfaces becomes diffusion-limited at high currents. A diffusion-limited loss of the Li inrted into an electrode particle at
a high rate of charge or discharge reprents a reversible loss of capacity. However, on charge/discharge cycling, changes in electrode volume, electrode-electrolyte chemical reactions, and/or electrode decomposition can cau an irreversible loss of capacity. Electrode-electrolyte chemical reactions that result in the irreversible formation of a passivating solid-electrolyte-interpha (SEI) layer on an electrode during an initial charge of a cell fabricated in a discharged state are distinguished from the irreversible capacity fade that may occur with cycling. The percent Coulombic efficiency of a single cycle associated with a capacity fade is
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The cycle life of a battery is the number of cycles until the capacity fades to 80% of its initial reversible value. Additional figures of merit of a rechargeable cell, aside from cost and safety, are its density (specific and volumetric) of stored energy, its output power P(q) = V(q)I dis for a given discharge current, and its calendar (shelf) life. The available energy stored in a fully Page 4 of 29
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