Cycle Life Enhancement in Cu-Sn 

Anodes 

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20020702 101 
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Army Research Laboratory 

Adelphi, MD 20783-l 197 

ARL-TR-2664 May 2002 

Cycle Life Enhancement in Cu-Sn 

Anodes 

Jeff Wolfenstine, Donald Foster, Jeffrey Read, and Wishvender Behl 

Sensors and Electron Devices Directorate 

Approved for public release; distribution unlimited. 
Abstract 

This technical report summarizes the work on two different approaches that have been used to 

increase the cycle life of active-inactive composites in the copper-tin system. The first approach is 

to reduce the particle size of the active-inactive composities to the nano-scale (400 run). The 

second approach is the addition of extra elements that segregate to grain/phase boundaries and 

increase the cohesive strength of the boundary. Both approaches significantly improved the 

capacity retention of a Cu6Sn5 alloy. A larger improvement in capacity retention was exhibited by 

the addition of lo-wt.% iron compared to the reduction of the particle size to the nano-scale. The 

volumetric capacity of the Cu6Sn5 alloy containing 10 wt.% iron at 100 cycles is almost three 

tunes the theoretical capacity of graphite. The Cu6Sn510 wt.% Fe material has potential as a 

replacement anode for graphite in lithium-ion batteries. 

ii 
Contents 

Introduction *.............................................................,..............,.......................*............................ 1 

Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*..................................... 1 

Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... 2 

Results and Discussion . ..*.*........................................................................................................... 3 

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........... 6 

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..*...................*.......... 7 

Report Documentation Page . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Figures 

1. X-ray Diffraction Pattern of the As-Precipitated Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

2. Transmission Electron Photomicrograph of Some Nano-scale Cu&rS Powders . . . . . . . . . . . . . . . 4 

3. Volumetric Capacity Versus Cycle Number for the 10 wt.% Fe, Nano-scale, Melted and 

Mechanical Alloyed Cu&ng Materials Cycled in the Voltage Range 0.0-l .O V and 

0.0-l 5 v . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

4. The Capacity of the Cu$ns Alloy having Nano-scale Particles and/or 10 Fe-wt. % as Function 

of Cu Content at 100 Cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

. . . 

ill 
Introduction 

Recently, there has been interest in the use of composites consisting of a lithium active metal (metal 

that alloys with lithium (Li), i.e., tin (Sn)) dispersed within an inactive matrix (i.e., copper (Cu)) as 

replacement anodes for graphite in Li-ion batteries [l-15]. Active-inactive composites are preferred over an anode consisting only of a lithium alloy, even though there is loss of specific capacity because of the extra weight of the inactive component because the cycle life of the alloy is very 

limited. Dispersing the alloy within a matrix reduces the mechanical stresses associated with Li 

insertion/removal during charging/discharging and thus improves the cycle life compared to the 

alloy alone. However, active-inactive composites still suffer from a restricted cycle life that limits 

their use in commercial and military applications. Thus, there is a need to develop ways to improve 

the cycle life of active-inactive composites so they can be used in practical applications. 

This technical report summarizes the work on two different approaches that have been used to 

increase the cycle life of active-inactive composites in the copper-tin (Cu-Sn) system. The fist 

approach is to reduce the particle size of the active-inactive composites to the nano-scale (cl00 

mn). The second approach, which has not attracted much attention in the battery community but is 

used in structural materials to improve the fracture toughness of the material, is the addition of 

extra elements, particularly elements that segregate to grain/phase boundaries and increase the 

cohesive strength of the boundary and thus, its resistance to fracture [M-21]. The results of both 

approaches will be compared to 1) graphite and 2) copper-tin active-inactive composites with micronsized particles. 

Material Selection 

For this investigation, the Cu&& alloy in the Cu-Sn system was chosen for the following reasons: 

1) It has been shown that as Li is added to Cu6Sns, it eventually decomposes to a Li-Sn alloy 

(active) surrounded by a Cu matrix (inactive) (i.e., forms active-inactive composite) [ll-141; 2) The 

volumetric capacity of a Cu6Sns alloy made by conventional melting [12] at 10 cycles was about 

twice the capacity for graphite, whereas for the Cu6Sns alloy at 25 cycles made by mechanical 

alloying [14], the capacity was about three times that for graphite. Thus, Cu& has potential as a 

replacement for graphite in Li-ion batteries if its cycle life can improved; and 3) there is experimental evidence for the Cu6Sns alloy in the micron-size particle range that as the particle size is decreased, an improvement in cycle life is exhibited [Et]. 

1 
Experimental 

Nano-scale Cu&$ powders were synthesized by a chemical method, which involved reducing a 

methanol solution of copper chloride (CuQ) and tin chloride (SnC&) (1.2: 1 volume ratio) with 

sodium tetrahydridoborate (NaBH,) in a 14 Molar sodium hydroxide (NaOH) solution under constant stirring at room temperature. Twice the amount of the NaBHa solution was added to the methanol 

solution containing the Cu and Sn ions to ensure complete reduction of the metal ions. Upon addition of the NaBH4 solution, a black precipitate was observed immediately. The black precipitate 

was washed with distilled water and filtered until the pH of the filtrate was the same as that of the 

distilled water. The precipitate was then dried at 100” C under vacuum. 

The second approach involved the addition of a third element, Fe, to a Cu$n, alloy. Iron was 

chosen as the additional element for the following reasons: 1) It has been shown theoretically based 

on a frost-principles approach that Fe will segregate to grain boundaries in N&AI and will enhance 

the room temperature cohesive strength of the boundaries [20]; 2) It has been shown that Fe additions to a Zn-22 wt.% Al alloy [21-231 segregated to grain boundaries and made these boundaries 

more resistant to sliding at elevated temperature. It was anticipated that Fe would segregate to grain 

boundaries in the CusSns alloy and would increase their resistance to fracture and thus improve 

cycle life. 

The material investigated in this study was Cu6Sn,+10 wt.% Fe. Elemental powders of Cu, Sn, and 

Fe in the correct ratio to form a Cu6SnS+10 wt.% Fe material were placed in a hardened steel jar 

along with several steel balls under an argon atmosphere containing less than 1 ppm of oxygen and 

water. The jars were shaken with a Spex mixer mill for 20 hours. After milling, the powders were 

cold pressed into pellets in a glove box under an argon atmosphere containing less than 1 ppm 

oxygen and water. The pellets were melted in a molybdenum (MO) crucible in the glove box at 

1100” C for about 24 hours, after which, the liquid was quickly poured onto a stainless steel pain to 

rapidly cool it. The quenched material was crushed, ground, and sieved to less than 45 pm. 

Powders from both approaches were characterized by x-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Powders for TEM were prepared by 

dispersing them in methanol, and while they were in an ultrasonic bath, a drop of the suspension 

was placed on a nickel grid covered with an amorphous carbon film. 

The cycle life of the Cu6SnS powders was evaluated in half cells at room temperature. Cu6SnS 

positive electrodes were prepared by mixing 85wt. % alloy powders, 5-wt.% carbon, and lo-wt.% 

polyvinylidene fluoride dissolved in N-methylpyrrolidinone. The mixture was coated onto a Cu 

substrate. The electrodes were dried under vacuum. Metallic lithium was used as the negative 

electrode. The electrolyte solution was 1 Molar Lithium Hexafluorophosphate:Ethlylene Carbonate/ 

Dimethyl Carbonate/Diethyl Carbonate (5:4:1 by volume). The cells were cycled at a constant 

current density of 0.1 mA/cm* between 0.0 to 1 .O V. After cycling, the structure of the alloys was 

investigated with x-ray diffraction. 

2 
Results and Discussion 

Figure 1 is an x-ray diffraction pattern of the Cu6Sns powders after precipitation. From Figure 1, it 

can be observed that the precipitate was single-phase Cu&ns. No other Cu-Sn phases or pure component (Cu or Sn) peaks were exhibited in the x-ray diffraction pattern. The average crystallite size 

calculated with the Scherrrer formula [25] was 10 nm. X-ray microanalysis on particles that were 

cold mounted and polished revealed that the Cu and Sn were uniformly distributed with a Cu:Sn of 

about l:l, which is in good agreement with the desired Cu:Sn of 1.2:1 for the Cu6Sn5 alloy. SEM 

analysis revealed that the particles exhibited an equiaxed morphology, with all particles less than 

100 run. The average particle size determined with the line intercept method [26] on more than 200 

particles was estimated to be about 40 nm. TEM confirmed that all particles were nano-scale 

(cl00 nm). Figure 2 is a TEM photomicrograph of some nano-scale Cu6Sn5 powders. It shows 

equiaxed particles with a size between 30 and 40 nm. 

The x-ray diffraction pattern for the Cu6Sn5+10 wt.% Fe material revealed mainly peaks corresponding to Cu&ns and to a lesser extent, those for elemental Fe. No other Cu-Sn, Cu, Sn, or Cu-Fe 

peaks were observed. Some very small peaks corresponding to FeSn and FeSn2 were observed. 

However, they were very much smaller than those for elemental Fe. From the x-ray diffraction 

data, it was concluded that the material is mostly single-phase Cu&ns with elemental Fe as a second phase. X-ray microanalysis with the SEM on particles that were cold mounted and polished 

2500 L Cu&nS 

Figure 1. X-ray Diffraction Pattern of the As-precipitated Material. 

As-ppt 

Ot.“““““““““““““““““““” 

0 IO 20 30 40 50 60 70 80 

28 

3 
Figure 2. Transmission Electron Photomicrograph of Some Nano-scale 

Cu6Sn5 Powders. 

revealed that the Cu and Sn were uniformly distributed with a Cu:Sn ratio of about 1.1: 1, which is 

in excellent agreement with the desired Cu:Sn of 1.2: 1 for the Cu$ns alloy. Iron was observed to be 

non-uniformily distributed. X-ray microanalysis revealed that the volume fraction of Fe converted 

to a wt.% was about 8% to 9%, which is in good agreement with the desired 10 wt.%. The average 

particle size of the Cu&++lO wt.% Fe material was about 6 pm. Preliminary TEM x-ray microanalysis confirmed a non-uniform Fe distribution within the particles. However, at present, it is 

not possible to specify the Fe location (i.e., along grain boundaries). 

The volumetric capacity of the nano-scale Cu&ns alloy (= 40 nm) (Approach 1) Cu$ns+lO wt.% 

Fe material (Approach 2) versus cycle number are plotted in Figure 3. Also shown in Figure 3 are 

data for graphite (theoretical capacity) [ll] and the Cu&ts alloy with micron-sized particles (information about the particle size is listed in parentheses) prepared by conventional melting (< 38 pm) 

[12] and mechanical alloying (~1 km) [14]. From Figure 3, several important points are noted. 

First, it can be observed that the extrapolated capacity approaches zero at about 45 to 50 cycles for 

the melted alloy and 80 to 85 cycles for the mechanical alloyed material. In contrast, at 100 cycles, 

the nano-scale Cu&ns and Cu&,+lO wt.% Fe material powders still have a significant amount of 

reversible capacity. This result shows that both approaches (nano-scale and Fe addition) have significantly improved the capacity retention of the Cu&-rs alloy. Second, the capacity of the C@ns4 
--&- 10 wt.% Fe 

-- ______...____ MeIt (< 38 p,) 

--- Mech Alloy (cl pm) 

--A-- Nano (40 nm) 

0 20 40 60 80 100 120 

Cycle number 

Figure 3. Volumetric Capacity Versus Cycle Number for the 10 Wt.% Fe, Nano-scale [ 151, Melted [12] and 

Mechanical Alloyed [ 141 Cu6Sn5 Materials Cycled in the Voltage Range 0.0-l .O V (10 Wt.%Fe, nano-scale and 

melted) and 0.0-1.5 V (mechanical alloyed). 

10 wt.% Fe material at 100 cycles is about 1.6 times the capacity that for the Cu$nS alloy having 

nano-scale particles, even though the particle size is about 150 times larger (6 pm versus 40 nm). It 

is believed that this effect is a result of the increased resistance to fracture of grain boundaries in the 

Cu6SnS material containing Fe compared the Cu&n, material without Fe. However, to confii this 

suggestion, a very detailed analytical study would be required. Third, the volumetric capacity of 

the nano-scale Cub& alloy at 100 cycles (~1450 n-Ah/ml) is almost twice, while the capacity for 

Cut&-10 wt.% Fe material at 100 cycles (~2320 mAh/ml) is almost three times the theoretical 

capacity of graphite (-850 mAh/ml) [ll]. Thus, both materials have potential as a replacement 

anode for graphite in Li-ion batteries. 

It is therefore of interest to speculate what the capacity might be for the Cu&nS alloy having both 

nano-scale particles and containing Fe. The gravometric capacity of the Cu6SnS alloy having nanoscale particles and with 10 Fe wt. % as function of Cu content (=55% Cu&$) at 100 cycles is 

shown in Figure 4. The data are taken from Figure 3. The dotted line presents the theoretical capacity, assuming the following decomposition reaction: 

4.4Li + Cu,Sn = yCu + L&$n (1) 

From Figure 4, several important points are noted. First, as expected, as the amount of Cu decreases, the theoretical capacity decreases. Second, as shown earlier in Figure 3, the Fe has a greater 

effect of capacity retention than reducing the particle size to the nano-scale. Third, assuming that 

the impurity and particle size effects are independent, which is probably valid in a first approximation, it can be observed that in this case, the predicted capacity of a nano-scale particle size Cu6SnS 

alloy with the Fe addition will approach the theoretical value, assuming that the reversible capacity 

is associated with the decomposition of Cu&& to Cu + Li&n. Thus, it is suggested that the use of 

a single method may not be sufficient and future work attempts to increase the cycle life of alloys 

should focus on the use of a combination of methods. 

5 
1000 

200 

0 

I I I 1 I 1 , I 8 I I I , I I 1 

-. 

-. 

- -. 

-. ‘. ‘. 4.4Li + Cu,Sn = yCu + Lid&n 

-- I -. -. 

-. -5 -. 

-. -. ‘. J ‘. -. Impurity + Particle Size ‘. 

-. 

-. -. 4 I 

‘----.--J-j 

‘. -. 

-. -. -. 

Impurity __) q ****=.._ 

-. 

Particle Size *Cl 

-. 

-. -. 

-. -. -. 

-. -. 

-. 

I I I I I I1 I,,, -. - I I 1 I I I --. 

0 20 40 60 80 100 

Cu (at.%) 

Figure 4. The Capacity of the Cu$ns Alloy Having Nano-scale Particles (particle size) And/Or 10 Fe-wt. % 

(impurity) as Function of Cu Content (=55% Cu&n,) at 100 Cycles. (The dotted line represents the theoretical 

capacity for the decomposition of Cu6Sn5 to Cu + L&$n.) 

Conclusions 

The results of this study can be summarized as follows: 

1) Nano-scale Cu6Sn5 powders (< 100 nm) were synthesized by a chemical method that involved reduction of the metallic ions with a NaBH, solution. 

2) An improvement in capacity retention can be obtained when the particle size of the Cu6Sn5 

alloy was reduced from the micron to the nano-scale range. The volumetric capacity of the 

nano-scale Cu6Sn5 alloy at 100 cycles is almost twice the theoretical capacity of graphite. 

3) A significant improvement in capacity retention of a Cu&n, alloy was exhibited by the 

addition of lo-wt.% Fe compared to the alloy of similar particle size but without the Fe 

addition. It is believed that this effect is a result of the increased resistance to fracture of 

grain boundaries in the Cu&Sn5 material containing Fe compared the Cu6Sn5 material without Fe. The volumetric capacity of the Cu6Sn5 alloy containing Fe at 100 cycles is almost 

three times the theoretical capacity of graphite. Thus, the Cu$n,-10 wt.% Fe material has 

potential as a replacement anode for graphite in Li-ion batteries. 

4) The Fe addition had a more significant effect on capacity retention than the reduction of the 

particle size to the nano-scale. For example, at 100 cycles, the reversible capacity is about 

1.5 times higher for the Ct.@n5 material containing Fe compared to the Cu6Sn5 material 

with the nano-scale particles. 
5) If the impurity and particle size effects are independent, it can be expected that the predicted capacity of a nano-scale particle size Cu&nS alloy with the Fe addition will approach 

the theoretical value, assuming that the reversible capacity is associated with the decomposition of Cu6SnS to Cu + L&,$n. Thus, it is suggested that the use of a single method may 

not be sufficient, and future work attempts to increase the cycle life of alloys should focus 

on the use of a combination of methods. 

Acknowledgments 

This work was performed under the Director’s Research Initiative Program (FYOl-SED-15) of the 

U. S. Army Research Laboratory. 

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14. Y. Xia, T. Sakai, T. Fujieda, M. Wada, H. Yoshinaga, J. Electrochem. Sot. 148 (2001) A471. 

7 
15. J. Wolfenstine, S. Campos, D. Foster, J. Read, W. K. Behl, Journal of Power Sources, in press. 

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24. F. A. Mohamed, SuqfI Znte@zce Anal. 31 (2001) 532. 

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26. M. I. Mendelson, J. Am. Ceram. Sot. 52 (1969) 443. 

8 
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13. ABSTRACT (Maximum 200 words) 

This technical report summarizes the work on two different approaches that have been used to 

increase the cycle life of active-inactive composites in the copper-tin system. The first approach 

is to reduce the particle size of the active-inactive composities to the nano-scale (cl00 nm). The 

second approach is the addition of extra elements that segregate to grain/phase boundaries and 

increase the cohesive strength of the boundary. Both approaches significantly improved the 

capacity retention of a Cu6Sn5 alloy. A larger improvement in capacity retention was exhibited 

by the addition of lo-wt.% iron compared to the reduction of the particle size to the nano-scale. 

The volumetric capacity of the Cu6Sn5 alloy containing 10 wt.% iron at 100 cycles is almost 

three times the theoretical capacity of graphite. The Cu6Sn510 wt.% Fe material has potential as 

a replacement anode for graphite in lithium-ion batteries. 

14.SuBJEmmRMS Anode, Li-ion, battery, capacity 15. NUMBER OF PAGES 

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