Ashlee N. Gordon Mentor: Dr. Quinton Williams 20 July 2018 Effect of Carbon Nanofibers on the Discharging Rate Performance of LiFePO4 Cathodes

Overview Motivation LFP as a solution? Summer goal w/ CNFs Battery fabrication Results Conclusions Future work

Motivation Need for portable energy Currently use LiCoO2 batteries Relatively low reversible specific capacity (140 mAh/g) Cobalt is toxic and expensive O2 poses safety concerns due to explosions (hover boards, cell phones) Do not reach their theoretical specific capacity (140 mAh/g) – how much energy can be held in material Relatively low specific capacity for discharge O2 provides an environment for fires to thrive Overcharging raises pressure inside the cell Overheating (thermal runaway)

LiFePO4 High theoretical specific capacity (160 mAh/g) Non-toxic Advantages Disadvantages High theoretical specific capacity (160 mAh/g) Non-toxic Inexpensive source materials Long cycle life Excellent thermal stability Relatively low electronic conductivity Slow Li-ion diffusion kinetics LiFePO4 has an olivine structure which means there is a one dimensional channel through which the lithium ions can be inserted/extracted (intercalation) LFP (~1 x 10-9 S cm-1) vs LCO is (~1 x 10^-4)

Summer Goal To optimize the overall performance of Li-ion rechargeable batteries using carbon-nanofibers as conductive additives in LiFePO4 (LFP) cathodes to improve: ionic diffusion electrical conductivity

Why carbon-nanofiber additives? High electronic conductivity via reactive inner and outer surface structure 1 Longer than CNTs, which improves electrical network in the cathode material 2 Less sensitive to interactions between neighboring particles than CNTs and other additives 3 High aspect ratio (ratio of length to width) Length improves electrical network in the cathode material versus CNTs

Cathode (on aluminum foil) Slurry for electrodes Cathode (on aluminum foil) LFP – 80% (active material) Polyvinylidene fluoride (PVDF, binder) – 10% Carbon black – 10% - 7% - 5% - 3% CNF – 0% - 3% - 5% - 7% Anode (on copper foil) Carbon graphite – 70% PVDF – 30%

Battery fabrication Step 1: Weigh out material for electrode Step 2: Mix slurry with vortex machine Step 3: Spread out using Dr. Blade Step 4: Bake in oven at 80˚C for 24 hrs Step 5: Cut electrodes using precision disc cutter Step 6: Weigh to find amount of active material (LFP) Step 7: Assemble in glove box Step 8: Test using Arbin Battery test station to track specific charge/discharge capacity, cyclability More specifically: during a discharge of electricity, the chemical on the anode releases electrons to the negative terminal and ions in the electrolyte through what’s called an oxidation reaction. Meanwhile, at the positive terminal, the cathode accepts electrons, completing the circuit for the flow of electrons. The electrolyte is there to put the different chemicals of the anode and cathode into contact with one another, in a way that the chemical potential can equilibrate from one terminal to the other, converting stored chemical energy into useful electrical energy. “These two reactions happen simultaneously,” Allanore says. “The ions transport current through the electrolyte while the electrons flow in the external circuit, and that’s what generates an electric current.” A chemical compound that ionizes when dissolved or molten to produce an electrically conductive medium

Discharge Capacity at Different C-Rates C-rate is the time is takes for the battery to completely charge/discharge relative to its specific capacity Current settings: 1C = 150 mAh/g Voltage Window: 2.5V-3.6V 5% CNF batteries show better SDC at low c-rate of 0.1C (10 hours) and at high c- rate of 5C (12 minutes) 5% CNF discharge profile shows improved specific discharge capacity versus 0% CNF (pristine-LFP) for both 0.1C and 5C *Average of 3 batteries each

LFP + 3% CNF cathode Current settings: 1C = 100 mAh/g Voltage window: 2.5- 3.6V 3% CNF batteries show slight improvement of SDC over 0% batteries *Average of 3 batteries each

C-Rate Testing 1 cycle is a complete charge and discharge Improved rate performance for 5% CNF batteries vs 0% CNF 5% CNF batteries maintained improvement after multiple cycles C-rate is the time is takes for the battery to completely charge/discharge relative to its specific capacity For this batch of batteries, 1C = 150 mAh/g since battery should not discharge below 2.5V*, it cannot reach the full capacity *Average of 3 batteries each

LFP + 3% CNF cathodes At low c-rate, 3% shows slight improvement At high c-rate, 3% loses capacity rapidly *Average of 3 batteries each

LFP + 3% CNF batteries did increase SDC at low c-rates, but capacity fades by last cycle LFP + 5% CNF batteries have the most promising results to increase SDC of LIBs and have good cyclability Conclusions

Acknowledgements Financial support from the REU site in Physics at Howard University NSF Award PHY 1659224 is gratefully acknowledged Dr. Quinton Williams Adewale Adepoju

Questions?