You might wonder why the lithium-ion cell voltages are so rigidly defined. Why can't we elevate the voltage peaks of NMC or LFP to harness more energy from the same material base? The answer is as much about chemistry as it is about safety & efficiency. The fixed voltages of these cell chemistries are determined by the 'electrochemical potentials' of the electrode materials they comprise. These potentials are the natural result of the materials' atomic structures & the energy levels at which they can comfortably accept & donate li-ions during the charge & discharge cycles.
Trying to exceed the inherent electrochemical threshold of the electrode materials in a battery means the possible decomposition of materials, unsafe operating conditions, and even an unwanted thermal runaway, where a battery could overheat and catch fire.
On the anode side, shown in blue in the image, we observe LTO marked at a low potential of around 1.55 volts & a specific capacity near 175 mAh/g. Graphite, a commonly used anode material, appears at a slightly higher potential, close to 0.1 volts, with a specific capacity approximately reaching 370 mAh/g. As we progress towards materials with a higher specific capacity, we see Silicon-Carbon composites (Si-C) with 2% silicon, showing a potential near 0.2 volts & a specific capacity around 550 mAh/g, and Si-C with 20% silicon, which maintains a similar potential while significantly boosting the specific capacity to nearly 1000 mAh/g. The Silicon anode presents itself at a potential of just around 0.5 volts, boasting a substantial specific capacity that exceeds 4000 mAh/g. The Li-Metal anode, often known for its high capacity, is plotted at a potential close to 0 volts, indicating its high reactivity & theoretical specific capacity that surpasses 3800 mAh/g.
In the cathode materials, colored orange, we see LFP with a potential of appx. 3.45 volts & a specific capacity near 170 mAh/g. The graph also showcases LMO & variants of NMC, as well as NCA, all of which are clustered together at higher potentials ranging from about 3.7 to 4.1 volts. The specific capacities of these materials vary, with LMO positioned near 140 mAh/g & the NMC/NCA group showing a wider range of capacities, typically between 160 to 200 mAh/g.
The values mentioned above are approximate & can vary due to specific material composition & testing conditions when measured against Li/Li+. They should be viewed as general guidelines rather than exact specifications. The final cell voltage is determined by the difference in potential between the anode & cathode.
As we progress, it is critical to respect the natural limits of these materials while continuing to innovate within the framework of what's chemically & physically possible.
Comments