We are all familiar with lithium-ion batteries (LiBs). They power everything from smart phones, laptops, medical devices, electric vehicles and even grid storage. With major companies like CATL, Northvolt, and Tesla currently scaling up their production of LiBs, it is permissible if you’ve forgotten that LiBs are in fact a relatively mature technology. To recap, a typical lithium-ion battery uses carbon (graphite) as an negative electrode and lithium-metal oxide (e.g., NCA, NCM, LCO) as a positive electrode. Using these materials, a LiB’s energy density tops out around 700 Wh/L. Companies are seeking higher energy for space-limited applications, especially electric vehicles, and thus tinkering with the materials inside traditional LiBs to do so. The question bears asking: What then, exactly, are they doing? What comes after LiBs?
There are essentially two major ways to increase a battery’s energy density and that lies in modifications made to either the positive or negative electrode material. These options are often spoke of in reference to next-generation batteries, or advanced Li-ion batteries. An example of one explored solution is to use silicon as a negative electrode material, either mixed with graphite or as a standalone. Silicon has a theoretical specific capacity of 4,200 mAh/g which is significantly higher than graphite at only 372 mAh/g and would thus increase the energy density of a traditional LiB. However, when lithium ions electrochemically react with silicon during the charging process, it volumetrically expands up to 300% which causes damage to the electrode and renders the battery useless after only a few cycles. For the positive electrode, increasing the nickel content due to its high energy when compared to other materials in the electrode (i.e., manganese, cobalt and aluminum) is believed to be the best route. Increasing the nickel content in a lithium-nickel-manganese-cobalt oxide (NMC) material up to a ratio of 8:1:1(Ni:Mn:Co)could increase the material capacity to exceed 185 mAh/g compared to the currently used 1:1:1 ratio at 154 mAh/g. While the higher nickel content increases the energy density, the thermal stability decreases. Also, fully oxidized Ni4+ (what you should get after charging) is reactive and thus creates unwanted side reactions with the electrolyte. Lastly, the cobalt and manganese elements are what hold the NMC layered oxide structure together, so decreasing their content would weaken the structural stability of the material.
So what comes after the above options for lithium-ion technology have peaked? There are a number of research projects and groups out there looking beyond lithium-ion but here are the big three: lithium-metal, solid-state, and lithium-air.
A lithium-metal battery is arranged in a similar manner to a traditional lithium-ion battery except that the anode material is made out of pure lithium. With a specific capacity of 3,680 mAh/g, a lithium metal electrode could increase a battery’s specific energy by 35 percent and the volumetric energy density by 50 percent. Owing to its high lithium-ion storage capacity, a much thinner electrode can be used compared to the traditional carbon anode, thus enabling a smaller form factor than that of today’s batteries. However, in order for this technology to succeed, researchers must overcome dendrite growth and the high reactivity of Li metal which results in low cycling efficiency and severe safety concerns respectively.
Solid-state batteries have gotten an increasing amount of attention recently with the large amounts of investment money going to companies focused on this technology. A solid-state battery is exactly what it sounds: a battery containing only solid components. Generally, this refers to the battery containing solid electrodes (lithium metal anode and high energy cathode) and a solid electrolyte. One of the unique features of the solid-state battery is the replacement of the volatile, flammable liquid electrolyte with a solid, non-flammable electrolyte. Common materials for the solid electrolyte include polymers, ceramics and glass. Aside from the safety benefits, solid-state batteries are expected to reach a higher volumetric energy density (Wh/L) than conventional lithium-ion cells, meaning you can pack
This plus the safety features afforded by a solid-state electrolyte are very attractive for the electric vehicle (EV) industry as EV fires and low driving range scare away potential customers. For example, a solid-state battery would increase the range of a Volkswagen E-Golf to approximately 460 miles compared with the present 180 km. So why haven’t these batteries taken the world by storm? Well, it is still early in the development phase. Solid-state batteries are expensive to manufacture and scientists are still working on increasing the cycle life of the battery to maintain performance over the 1,000 - 1,200 cycles required of EVs. Lastly, by replacing the liquid electrolyte with a heavier solid electrolyte, the weight of the cell goes up thus decreasing the specific energy density (Wh/kg). This diminishing of specific energy density can be countered by the higher volumetric energy density, but this is something that remains to be proven in large scale tests.
Way out in the future beyond even solid-state batteries come lithium-air batteries. A lithium-air cell creates voltage from the availability of oxygen molecules (O2) at the positive electrode. O2 reacts with the positively charged lithium ions to form lithium peroxide (Li2O2) and generate electric energy. The lithium-air battery is coveted because it has a practical specific energy density of about 1,700 Wh/kg, which is almost 10 times that of lithium-ion batteries and almost on par with gasoline. This technology is currently in its infancy with a laundry list of issues. Li2O2 is a very poor electron conductor. If deposits of Li2O2 grow on the electrode surface that supplies the electrons for the reaction, it dampens and eventually kills off the reaction and therefore depletes the battery’s power. Another issue is the amount of oxygen it needs to operate. Currently, only pure O2 can be used and normal air is not sufficient to provide the energy densities that make it so attractive.
Liu, B., Zhang, J.-G., & Xu, W. (2018). Advancing Lithium Metal Batteries. Joule, 2(5), 833–845. doi:10.1016/j.joule.2018.03.008