800 600 400 200 0 C ap ac it y (mA h/ g) 100 80 60 40 20 0 Cycle Number
Sb@Ni Cu0.27Sb@Ni Cu1.86Sb@Ni Sb@Cu Cu0.27Sb@Cu Cu1.86Sb@Cu
Exploring and mitigating failure modes in anodes for Li-ion batteries
Maxwell C. Schulze and Amy L. Prieto
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
How can batteries store more energy?
Electrodeposition of thin film Cu-Sb anodes
Electrochemical performance of Cu-Sb anodes is substrate dependent
1) J. B. Goodenough and K.-S. Park, The Li-Ion Rechargeable Battery: A Perspective , J. Am. Chem. Soc., 2013, 135, 1167–1176.
2) L. Fransson, J. T. Vaughey, R. Benedek, K. Edström, J. O. Thomas and M. M. Thackeray, Phase transitions in lithiated Cu2Sb anodes for lithium batteries: an in situ X-ray di︎raction study, Electrochem. Comm., 2001, 3, 317–323. raction study, Electrochem. Comm., 2001, 3, 317–323. 3) J. M. Mosby and A. L. Prieto, Direct Electrodeposition of Cu2Sb for Lithium-Ion Battery Anodes, J. Am. Chem. Soc., 2008, 130, 10656–10661.
4) E. D. Jackson and A. L. Prieto, Copper Antimonide Nanowire Array Lithium Ion Anodes Stabilized by Electrolyte Additives, ACS Appl. Mater. Interfaces, 2016, 8, 30379–30386.
We use electrodeposition to produce Cu-Sb anode materials we are interested in studying. By careful selection of the deposition solution composition and the potential applied between the counter and reference electrodes, we produced thin films of our desired material directly on metal foil substrates (left).3 The films are characterized using X-ray
diffraction (XRD) and scanning electron microscopy (SEM) both before and after cycling them in Li-ion half cells (right) to gain understanding about how the materials fail during cycling.
Rechargeable Li-ion batteries store energy by reversibly storing Li-ions (Li+) at different
electrochemical potentials in negative (anode) and positive (cathode) electrodes. The cell voltage (difference in potential between electrodes) and the
capacity (amount of Li+ in each electrode) determine
the energy the cell can store.
Thus, one way to increase the energy that a battery stores is to use an electrode material with greater capacity per unit mass (mAh/g) or per unit volume (mAh/cm2). Many new materials exist that have
capacities greater than graphite, the anode used in today’s Li-ion batteries. However, many of the new materials exhibit limited useful lifetimes due to failure modes that we aim understand and mitigate to produce high capacity batteries that are commercially viable.
Sb
Li
3Sb
660 mAh/g 4418 mAh/cm3 poor conductivity 129% volume changeC
6Li
C
6 372 mAh/g 800 mAh/cm3 good conductivity 10% volume changeCu
2Sb
Li
3Sb +
2Cu
323 mAh/g 2730 mAh/cm3 improved conductivity ~90% volume changeAlloying material: antimony
Conversion material: copper antimonide
Intercalation material: graphite
Next generation anode materials
Metal Foil Substrate Reference Electrode Counter Electrode Deposition Solution film or particles of active material cracking, pulverization, and delamination Failure mode: volume changes during cycling
What is still needed to make these anodes commercially viable?
nickel substrate mechanically robust interface Sb Film Ni Foil Sb Film Ni Foil 300 nm Sb Cu Si Cr Ni Cu Foil Sb Film Cu Foil Cu2Sb Sb Film 300 nm Cu-Sb Cu Si Cr copper substrate mechanically unstable interface Kirkendall voids
Electrochemical cycling of Sb on Cu drives interdiffusion
(Top) Voltage vs. capacity traces of Sb thin film on a Cu substrate. (Middle) Differential analysis of the top data shows the electrochemical transformation of Sb to that of Cu2Sb over only 6 cycles. (Bottom) XRD measurements of the anode before and after cycling confirms the structural transformation from Sb to Cu2Sb.
The interdiffusion between Sb-containing thin films and Cu substrates to form Cu2Sb is confirmed by XRD. The process of interdiffusion creates Kirkendall voids that mechanically weaken the film-substrate interface and exacerbate film delamination. This weak interface results in Cu-Sb thin film anodes of any Cu-Sb composition to have cycle lifetimes inferior to those on Ni substrates.
The lack of interdiffusion between Sb-containing thin films and Ni substrates results in a film-substrate interface that is resilient to the volume changes of the Sb as it alloys and dealloys with Li+.
This strong interface results in Cu-Sb thin film anodes of any Cu-Sb composition to have cycle
lifetimes superior to those on Cu substrates. 2 um Working Electrode (Cu-Sb Thin-film) Separators and Electrolyte Counter/Reference Electrode (Li Foil)
Li-ion half cell
±Li
±3Li
±3Li
Reference 1
Reference 2
Mechanical degradation still plagues the cycle lifetimes of alloy and conversion anodes. Nano-structuring of these materials has been shown to help alleviate such degradation by providing space for active material to expand and contract. However, another battery failure mode can be exacerbated by nano-structuring: excessive growth of the solid-electrolyte-interface (SEI) layer as seen above as a residue coating a nanowire array anode after cycling in a Li-ion battery. Strategies to mitigate this failure mode include the use of electrolyte additives to passivate SEI layer growth and extend cycle lifetimes (shown left).4 Ultimately, the design of commercially viable batteries with increased
energy density needs to address all failure modes concurrently.
1 µm 1 µm Nanowires of Cu2Sb resist m e c h a n i c a l d e g r a d a t i o n during cycling High surface area results in excessive g r o w t h o f SEI layer
We would like to thank Dr. Pat McCurdy (Central Instrumentation Facility at CSU) for assistance with the SEM–EDS. Funding for this research is provided through NSF SSMC grant #1710672.
2.5 0.0 V vs. Li/Li + -1.5 1.5 dQ /d V ( mA h/ V ) 1.2 1.1 1.0 0.9 0.8 0.7 0.6 V vs. Li/Li+ C ou nt s 45 40 35 30 25 20 2Theta (degrees) 100x10-6 80 60 40 20 Capacity (Ah) Cycle1 Cycle6 Pre-cycled Post-cycled Cu2Sb Sb Sb Sb Cu2Sb Cu2Sb Cu2Sb Cu2Sb Cu2Sb Cu2Sb * lithiation delithiation delithiation lithiation
