Design and Testing of Prelithiated Full Cells with High Silicon Content

Abstract

Prelithiation is a strategy of increasing importance for high energy density, long cycle life cells. This paper provides a thorough understanding of the implications of prelithiation on cell design and a phenomenological understanding of the behavior of prelithiated negative electrodes in full cells. In a first part, an idealized electrode stack model is derived showing the variation of energy density with prelithiation. Two regimes are identified, the first where prelithiation allows increased energy density by compensating the irreversible capacity of the negative electrode and a second where further prelithiation provides a lithium reservoir to compensate ongoing cycling losses. In a second part coin and cylindrical full cells are used to demonstrate the two regimes. Full coin cells are used to show the impact of the lithium reservoir and the impact of the coulombic efficiency of the negative electrode on the cycle life of a prelithiated cell. Cylindrical 2Ah cells are used to demonstrate the impact of accurate and repeatable roll to roll prelithiation combined with an engineered Si alloy. A cylindrical cell with a prelithiated negative electrode containing 55 wt% Si alloy demonstrated 80% capacity retention at 500 cycles and a coulombic efficiency of over 99.9% up to 700 cycles.

Insights on the cycling behavior of a highly-prelithiated silicon–graphite electrode in lithium-ion cells

“…That is, once the cell is prelithiated, capacity could be maintained stable at a much lower consumption rate of the Li + from the reservoir than initially anticipated. This is conceptually similar to observations reported by Chevrier et al [7] in which a prelithiated cell presented higher coulombic efficiency than the baseline even after the exhaustion of the reservoir. These behaviors are rationalized below.…”

“…During the first charge of this re-built cell, a coulombic efficiency of 88.5% is achieved, compared to 77.1% exhibited by the baseline cell. The first cycle of a prelithiated cell will generally have the efficiency limited by the cathode, justifying the relatively low value [7]; after the initial charge, the positive electrode is unable to accept a fraction of the extracted Li + at reasonable rates due to kinetic limitations [15]. This extra lithium will then join the reservoir at the anode, amounting to a total of ∼0.81 mAh cm −2 .…”

“…This is clearly not what happens. Reservoir consumption often manifests as a sudden increase in the rate of capacity loss [7], while the prelithiated cell in figure 3 continues to perform unperturbed by such sudden changes. A logical explanation is that, somehow, prelithiation also changes the intrinsic rate of fade of an electrode pair.…”

“…From a materials perspective, silicon has attracted considerable attention because it is capable of storing roughly 10 times more charge than graphite, which is the major anode constituent of commercial cells. Therefore, electrodes containing silicon in various amounts and forms have been the subject of extensive investigations in the past decade [3][4][5][6][7][8][9][10][11].…”

“…Higher levels of prelithiation can be used to create a Li + reservoir in the anode that is gradually released into the cell to reduce the magnitude and rate of capacity loss. Note that this second scenario requires higher anode/cathode capacity ratios, which can partially offset the energy gains brought about by silicon [7].…”