Batteries and a Sustainable Modern Society

67 A fter outlining the urgent need for realization of clean electrical energy generated by the sun for a sustainable modern society and the constraints of electrochemistry for this realization in a low-cost, safe rechargeable battery of long cycle life, recent advances in materials chemistry are outlined that provide an optimistic view that this realization may be possible in the relatively near future. All life on Earth is sustained by the conversion of solar energy into chemical energy by plants. The chemical energy stored in plants is delivered to mobile life by metabolic processes and to society by man as the heat of combustion with its attendant emission of polluting gases. Modern society has become dependent on the solar energy stored over millennia in fossil fuels; this energy store has allowed the exploitation of the resources of the planet Earth in a throwaway economy, which is not sustainable without recycling of these resources. The combustion of fossil fuels is not recyclable, and the attendant massive emission of gases contributes to global warming and can choke the inhabitants of large cities. Although the energy from fossil fuels can be supplemented by nuclear energy that delivers heat without the gases of combustion, nuclear energy is neither clean of pollutants nor as convenient as the energy stored in liquid or gaseous fuels. In summary, a modern society is not sustainable unless means other than plants are developed to harvest and store the sun’s energy and to deliver that energy as clean electric power without the pollutants from hydrocarbon combustion or the thermodynamic constraints on the efficiency of conversion of heat energy. Harvesting of the sun’s radiant energy by other means than by plants is possible with photovoltaic cells that convert it to electric power; nature converts it to hydropower and wind. The mechanical energy of hydropower can be stored in dams for conversion into electric power and windmills can convert wind energy into electric power. The heat delivered by nuclear fission is also transformed into electric power. The constant electric power delivered by nuclear energy needs to be stored for delivery to a variable demand or only used for the constant component of demand. On the other hand, radiant-solar and wind energies are diffuse and variable on diurnal and seasonal time scales. Although the electric power delivered by photovoltaic cells and windmills may be locally collected and transported over long distances, it needs to be stored for delivery as clean electric energy to a variable demand that may be either portable or stationary, distributed or centralized. A convenient long-term energy store of electrical energy as chemical energy that is delivered back as electrical energy is the rechargeable battery; the efficiency of the energy conversions in a battery is not constrained by thermodynamics other than by the heat loss associated with the internal battery resistance. Although the energy stored in the electrodes of a rechargeable battery is less dense and less versatile than the energy stored in a fossil fuel, its delivered energy is clean and can be portable. The challenge to the material chemist/engineer is to develop, with environmentally friendly materials, rechargeable batteries of high energy density that are safe and efficient with a long cycle and shelf life at a cost low enough to be commercially viable. There are three principal markets for a rechargeable battery: (1) powering portable hand-held devices, (2) powering electric road vehicles, and (3) stationary distributed or centralized electrical energy storage to supplement energy storage in the grid. Powering of handheld devices does not compete with fossil fuels, which is why today’s Li-ion batteries are ubiquitous. Powering electric vehicles for road transportation must compete with the internal combustion engine powered by a liquid fuel, normally a fossil fuel, of high energy density. The hidden cost to the environment and to the economy for securing access to the fossil-fuel sources are not apparent to the individual customer at the gasoline pump. Increasing safely the volumetric energy density of a battery cell beyond that of a Li-ion battery at temperatures to −20°C is critical for batteries that power an electric vehicle. These batteries store and deliver dc electric power, so they are charged more efficiently by electric power from solar energy than from wind energy. The ac power from wind energy is more efficiently stored directly into the grid than in a rechargeable battery. For stationary electrical energy storage, the amount and cost, including shelf and cycle life, of energy stored in a single charge is more critical than the volumetric energy density and operation at low temperatures.