Specific energy simply represents the product of the specific charge (in Ah/kg) and the average voltage of an electrochemical cell during discharge. The highest specific energy gives the combination of metallic lithium with oxygen. In fact, many scientists argue that oxygen can be taken from air and, thus, its mass does not need to be considered in terms of specific charge. If the calculation is done very optimistically, assuming the ultimate reaction 2Li + 1/2O2 = Li2O and electrochemical potentials under standard conditions, such a lithium–air cell is characterized by a theoretical potential difference of about 4 V and a specific energy of ca. 15’000 Wh/kg. Obviously, this is not true because the assumed reaction product, Li2O, is stored inside the cell and, therefore, the mass of oxygen needs to be accounted for, reducing the hypothetical specific energy to ca. 7’000 Wh/kg. Furthermore, research revealed that the final reduction product is Li2O2 rather than Li2O, which further reduces the theoretical specific energy to ca. 4’000 Wh/kg. Besides that, the thermodynamical cell voltage of the lithium-oxygen couple is slightly below 3 V in a non-aqueous environment, which brings down the numbers to the theoretical upper limit of specific energy of ca. 3’000 Wh/kg. Considering, finally, that many inactive auxiliary cell components are required, and that these, in first approximation, can be accounted for by dividing the theoretical specific energy based on the active materials by a factor of four, the expected specific energy of a hypothetical industrial lithium–air battery would be slightly below 1’000 Wh/kg. Similar considerations must be made for all other relevant battery systems to truly assess their ‘real-life’ potential.
Based on this type of considerations we developed an energy-cost model, which helps us to find the relationship between cost and energy density for different battery chemistries, and enables us to predict the most promising material combinations. The possibility of Li-ion batteries to operate at higher charging voltages (>4.2 V) than commercial cells on the market today will allow to extract higher amounts of charge without compromising coulombic and voltage efficiencies. In our opinion, this is the only way to significantly increase both gravimetric and volumetric energy densities for future lithium-ion battery systems. If a battery, in the best case, should last 10 years with about 300 cycles per year and 80% charge retention at the end of its life-time, a coulombic efficiency of at least 99.99% is needed. All active materials for positive electrodes, except for the Li-insertion compounds, are far from reaching such efficiencies today. Among the wide variety of proposed positive electrode materials only few show sufficient potential for commercialization, and, clearly, Li-rich and Ni-rich positive materials are definitely the winners, with Li–S and Na-ion emerging as contestants due to the low cost and abundance of their key components. As no further significant improvements in gravimetric/volumetric energy density and cost can be achieved through new battery chemistries, engineering efforts, targeting cost reduction and safety assurance, will most likely be the main driving forces behind future rechargeable battery development.
Our energy-cost model underlines the importance of considering full-cell configuration, i.e., the pairing of commercially relevant electrodes in the same cell, for arriving at reasonably reliable energy estimates, instead of continuing half-cell based research. We similarly want to highlight the importance of the full-cell perspective when investigating other cell parameters such as power, cycle-life, safety, etc., because many performance-related issues, such as, for example, transition metal leaching, are not evident in a commonly used half-cell configuration.