Batteries are devices that convert stored chemical energy into useful electrical energy. A battery may be thought of as a clever variant of a standard exothermic chemical reactor that yields chemical products with lower energy content than the chemical reactants. In such a chemical reactor, the overall chemical reaction proceeds spontaneously (possibly requiring a catalyst and/or elevated temperature) when the reactants are brought into physical contact. In a battery, the overall chemical reaction is divided into two physically and electrically separated processes: one is an oxidation process at the battery negative electrode wherein the valence of at least one species becomes more positive, and the other is a reduction process at the battery positive electrode wherein the valence of at least one species becomes more negative.
The battery functions by providing separate pathways for electrons and ions to move between the site of oxidation and the site of reduction. The electrons pass through the external circuit where they can provide useful work, for example power a portable device such as a cellular phone or an electric vehicle. The ions pass though the ionically conducting and electronically insulating electrolyte that lies between the two electrodes inside the battery. Therefore, the ionic current is separated from the electronic current, which can be easily controlled by a switch or a load in the external circuit. When a battery is discharged, an electrochemical oxidation reaction proceeds at the negative electrode and passes electrons into the external circuit, and a simultaneous electrochemical reduction reaction proceeds at the positive electrode and accepts electrons from the external circuit, thereby completing the electrical circuit. The change from electronic current to ionic current occurs at the electrode/electrolyte interface. Faradayâ€™s Law, which describes the quantitative proportional relationship between the equivalent quantities of chemical reactants and electrical charge, governs this change. When one attempts to recharge a battery by reversing the direction of electronic current flow, an electrochemical reduction reaction will proceed at the negative electrode, and an electrochemical oxidation reaction will proceed at the positive electrode.
The common non-rechargeable (throwaway) alkaline battery uses a zinc negative electrode, a manganese dioxide positive electrode, and an aqueous alkaline electrolyte. The overall battery cell discharge reaction is Zn + MnO2 + H2O = ZnO + Mn(OH)2. For this chemical reaction the free energy of the reaction products is lower than the free energy of the reactants, and therefore the reaction proceeds spontaneously. During battery cell discharge the electrochemical oxidation reaction at the negative electrode is Zn + 2OH- = ZnO + H2O + 2e-, and the electrochemical reduction reaction at the positive electrode is MnO2 + 2e- + 2H2O = Mn(OH)2 + 2OH-. One can use the well-known thermodynamic free energy change of the chemical reaction Zn + MnO2 + H2O = ZnO + Mn(OH)2 along with the Faraday Law to calculate the observed voltage of this cell, which is 1.5 volts.
The modern rechargeable lithium-ion battery uses a lithium-carbon negative electrode, a cobalt dioxide positive, and an electrolyte that contains a lithium salt dissolved in an organic solvent. The overall battery discharge reaction is LiC6 + CoO2 = C6 + LiCoO2. As for the throwaway alkaline battery, the free energy of these reaction products is lower than the free energy of the reactants, and therefore the reaction proceeds spontaneously. During battery cell discharge, the electrochemical oxidation reaction at the negative electrode is LiC6 = Li+ + C6 + e-, and the electrochemical reduction reaction at the positive electrode is CoO2 + Li+ + e- = LiCoO2. One can use well-known thermodynamic free energy change of the chemical reaction LiC6 + CoO2 = C6 + LiCoO2 along with the Faraday Law to calculate the observed voltage of this cell, which is 4.0 volts.
In the case of the rechargeable battery, the electrochemical oxidation-reduction reactions are reversible at both electrodes. For example, when the battery is recharged, the overall electrochemical reduction reaction at the negative electrode is identical to the electrochemical oxidation reaction that proceeded at the negative electrode when the battery was discharged, only written in reverse.
In the case of the non-rechargeable battery, when one attempts to recharge the battery by reversing the direction of electron current flow, at least one of the electrochemical oxidation-reduction reactions is not reversible. For example, when the battery is charged, the overall electrochemical reduction reaction that proceeds at the negative electrode may not be the reverse of the electrochemical oxidation reaction that proceeded when the battery was discharged. For example, metal oxidation might be the sole oxidation reaction during battery discharge, whereas gas formation might be a significant reduction reaction during battery recharge (a potentially dangerous condition!).
An added requirement for a well-behaved (i.e., long-lived and safe) rechargeable battery is that not only must the electrochemical oxidation-reduction reactions be reversible; they must also return the electrode materials to their original physical state. For example, if rough or filamentary structures are formed after repeated charge-discharge cycles, then there may result unwanted electrode growth and subsequent electronic contact between the battery electrodes, i.e. an internal short circuit that rapidly discharges the battery. Because of these requirements, the development of a well-behaved rechargeable battery is significantly more difficult than the development of a non-rechargeable battery.
As described above, the three fundamental components of a battery are the negative electrode (usually identified as the anode, where anodic oxidation proceeds when the battery is discharged); the positive electrode (usually identified as the cathode, where cathodic reduction proceeds as the battery is discharged); and the electrolyte (which is ionically conducting and electronically insulating). Now, let us examine in more detail each of these three battery components. We will focus on the components for the rechargeable lithium-ion battery because it and related batteries with non-aqueous electrolytes are considered to be the most promising technology for powering electric vehicles, hybrid-electric vehicles, and plug-in hybrid vehicles.
The anode must have a low mass per unit of charge delivered so that a high-energy battery can be developed, support high oxidation and reduction rates to allow rapid battery charge and discharge, exhibit a low electrochemical potential so that a high-voltage battery can be developed, show high coulombic (current) efficiency so that unwanted side reactions do not occur, be environmentally benign and safe, exhibit very long lifetime with little performance decay, and be low cost. To support high reaction rates that are suitable for vehicle applications, the anode must have a high surface area, and therefore anodes are typically fabricated using LiC6 carbons in the form of small particles. Also, the anodes must be designed to provide intimate contact between the electrolyte, the LiC6 carbon, and a so-called current collector (typically a thin copper sheet) so that electrons and ions have ready access to the sites of electrochemical reactions. This requirement usually leads to the addition of an extra carbon phase to enhance electronic conduction. Finally, the anode must not fall apart as the battery is charged and discharged, therefore a polymeric binder is used to hold the anode together. All of these requirements are very demanding and often conflict with one another. So, it is a very difficult task to arrive at an optimal set of materials and a suitable design for a modern, high-performance lithium-ion battery anode. The BATT Program conducts sophisticated fundamental and focused anode research to do exactly this, as well as explore the use of alternative anode materials such as lithium alloys.
Similarly, the cathode must have a low mass per unit of charge delivered so that a high-energy battery can be developed, support high oxidation and reduction rates to allow rapid battery charge and discharge, exhibit a high electrochemical potential so that a high-voltage battery can be developed, show high coulombic (current) efficiency so that unwanted side reactions do not occur, be environmentally benign and safe, exhibit very long lifetime with little performance decay, and be low cost. The requirements for the cathode parallel those of the anode, as described above. In addition, because the most promising cathode materials are brittle ceramics composed of metal oxides, they are susceptible to cracking and other forms of physical degradation as lithium ions are inserted and extracted. The BATT Program conducts leading cathode research to address these difficult problems.
The electrolyte must have high ionic conductivity to allow the development of a high-power battery, be stable at both the high potential of the battery cathode and the low potential of the battery anode, be compatible with a physical separator (usually a porous polymeric material) that prevents physical or electronic contact between the anode and cathode, adequately wet the anode, cathode, and separator, be environmentally benign, have low vapor pressure, and be low cost. The BATT Program conducts leading electrolyte research to address these difficult problems.
In addition, the BATT Program carries out comprehensive testing and diagnostic characterization of experimental batteries, and develops extremely sophisticated mathematical models of advanced batteries to not only help design next-generation batteries but also guide research to develop new materials and battery components.