The mostly widely used lithium (Li) batteries, including lithium-ion (Li-ion) batteries, are activated via organic solvent based electrolytes that are highly flammable. The flammable electrolyte can potentially impose impediments to large-format battery device implementation because, under abuse conditions (e.g., internal short circuit, overcharge, crush, etc.), Li batteries can be hazardous due to heat generation which can potentially lead to toxic/flammable material release, fire and over-pressurization/explosion [1]. These hazards represent major concerns for power source platforms populated by personnel, sensitive equipment and/or vital structures. Recent high-profile incidents involving energy dense batteries (e.g., the Boeing 787 Dreamliner battery fire incident [2]) have increased public awareness of battery safety and resulted in intensified scrutiny for the processes used to develop, manufacture, certify and transport Li batteries.

Here we present ADA efforts in the development of a Li battery technology comprised from a nonflammable, high performance, room temperature ionic liquid (RTIL)-based or hydrofluoroether (HFE)-based electrolytes and a nano-engineered, atomic layer deposition (ALD) functional material coated, high voltage, high energy density cathode (i.e., high voltage LiCoO₂ or HV-LCO). We demonstrate that such combinations can afford a Li battery technology with an unprecedented high energy density (over state-of-the-art Li-ion or Li batteries), high rate capability, a wide operation temperature range, long cycle life at room temperature (RT) and temperature extremes and safety.

Figure 1 shows electrolyte flammability test results. A lighter was used to ignite the electrolyte in a watch glass dish in open air. An organic solvent based, Li-ion battery baseline (conventional) electrolyte was ignited and self-burning when the ignitor was moved away. Each of the RTIL- and HFE-based electrolytes did not ignite and, no self-combustion was observed.

Figure 2 shows discharge voltage profiles of Li/HV-LCO cells with RTIL- and HFE-based nonflammable electrolytes as a function of temperature. The cells were cycled between 3-4.6V. At RT, the cells delivered a high specific capacity/energy (> 200 mAh/g and > 800 Wh/kg, cathode active weight). At 50°C, the cells delivered a slightly higher (1-3%) specific capacity than RT. At -20°C, the cells were cycled at C/10 rate and delivered ~75% of the capacity at RT (~155 mAh/g). At -40°C, the cells were cycled at C/20 rate. The HFE-based electrolyte cell delivered a much higher specific capacity (66.5 mAh/g) than the RTIL-based (negligible).

Figure 3 shows excellent rate capability of the RTIL-based electrolyte with 85% capacity retention at 10C rate.

Figure 4(a) shows high voltage cycle life between 3-4.6V at RT for pristine LCO and HV-LCO based Li cells using the RTIL-based electrolyte. The pristine LCO cells experienced a rapid capacity fade while the HV-LCO cells demonstrated excellent cycle life stability with > 92% capacity retention after 100 cycles. We attribute the excellent HV-LCO high voltage cycle performance to the ALD functional material coating on LCO; this coating effectively suppresses the degradation root cause, i.e., the Co⁴⁺ dissolution and hexagonal to monoclinic phase transition [3].

We also evaluated cycle life at an extreme high temperature of 75°C. Figure 4(b) shows high voltage cycle life test data for pristine LCO and HV-LCO based lithium cells using RTIL-based electrolyte with additives. The cells were cycled between 3-4.5V. The pristine LCO based cell still faded much more quickly than the HV-LCO based cell; the latter exhibited an extraordinary high voltage high temperature cycle resiliency (97% capacity retention after 30 cycles, vs. 75% of the pristine LCO) even at the extreme high temperature of 75°C. We attribute the outstanding cycle stability at 75°C to the synergistic benefits of nano-engineered cathode and passivation protection from the electrolyte additives.

Our study highlights the importance of combining high performance, safe electrolyte with nano-engineered cathode (e.g., ALD nano-engineered LCO) to render the resultant lithium battery with unprecedented energy density (gained from the high voltage operation), high rate capability, safety (nonflammability) and robust cycle life over a wide temperature range particularly at the extreme high temperatures (e.g., 75°C). We demonstrate that our Li battery technology is ideally suited for many high energy density and safety demanding applications, such as, electric drive vehicles, consumer electronics, medical, space and military applications.

References

1. Peter Roth and Christopher J. Orendorff, Electrochem. Soc. Interface, Summer 2012, page 44.
2. “FAA Grounds Boeing 787 Dreamliner for Battery Problems”, Aviation International News, January 2013
3. Akira Yano, Masahiro Shikano, Atsushi Ueda, Hikari Sakaebe, and Zempachi Ogumi, Electrochem. Soc., 164, A6116 (2017).