Good question and not a straightforward answer. It depends on quite a few things...
Firstly it depends on what type of Lithium chemistry you are talking about. Some types of cell are optimised for power delivery (highest power from the smallest, lightest cell). Some are optimised for energy delivery (the most energy stored per cell). Some are optimised for safety (no metallic Lithium or dangerous oxides that promote incendiary-bomb-like fires).
Many of these optmisations have an impact on the lifespan of the cells, both in their "calendar life" and their "cycle life". Today, let's start with the issue of calendar life.
Calendar life is a measure of how long it takes for a cell to self-destruct due to chemical impurities that cause side reactions to the desired ones. These side reactions gradually cause the cell to die and they start happening the day the cell is made. Nothing you can do to stop it, but you can slow it down. That's why AA cells and so on in the shops have a "best before" date stamped on them. Alkaline cells have a calendar life of about 4 years.
Lithium ion cells generally have quite long calendar life of over 10 years. But it depends on the chemistry and how they are stored. Lead acid batteries like to be stored in a cool place, fully charged and topped up regularly. Lithium ion cells like to be stored in a cool place and at a low state of charge (30-50%). But you have to be careful that they do not become over discharged (by attached monitoring chips or other circuits). They don't like being stored on charge all the time and in a hot place.
Unfortunately, that's often the kind of treatment they get. Laptop batteries are notorious for short calendar life of just 2-3 years. This is due to poor storage conditions. They are installed in the machine when used on the mains all the time. The cheap chargers keep the battery on charge the whole time (even when the laptop is turned off) and the heat from the motherboard can be extreme (45-55C locally inside the unit, near one or more cells of the battery pack). It would be better to half discharge the battery, take it out and keep it somewhere cool until needed. But then who does that with a laptop? It's very inconvenient to put the battery back in and charge it before needing to use the machine away from the mains. So people "abuse" the battery for convenience sake and complain when it dies even though they haven't used it for many real discharge cycles.
Some Lithium ion cells are more sensitive to storage conditions (especially temperature) than others. Lithium Iron Phosphate cells (like the Winston ones I'm using) are relatively "ok" with warmer temperatures. It does not have as big an impact on their life as for some Lithium metal oxide cells, which degrade more severely with heat. These metallic oxide cells are historically the types of cells used in laptops, so not a good combination... Cells that don't like heat, stored permanently in a warm / hot place.
It's also why the sealed lead acid batteries in my computer UPS (uninterruptable power supply) used to die every two years... It gets hot (35C) in the box where they are housed. If I keep the batteries outside the box and extend the wires into the box, the batteries last for at least 4-5 years. The culprit was temperature. After all, the batteries never discharged as they were for emergencies only.
So... My Lithium home energy battery bank is kept in a cool part of the house and I keep it shaded from the sun. The inverter is positioned so that it does not blow warm air at it. At temperatures ranging from 17-23C there (recorded by the charge controller) the battery pack should last for well over 10 years. Nobody knows for sure, because they've not been in production long enough to find out but I'll be one of the first to know!
Most testing of calendar life is done by deliberately running the battery at higher temperatures in "accelerated testing" and then estimating the life at lower temperatures, assuming that all the chemical reactions slow down in a linear way with temperature.
The other aspect to calendar life is storage charge level. As mentioned, different types of cells like to be stored at different charge levels. Lead acid likes to be stored fully charged and Lithium likes to be stored at 30-50% charge. This makes Lithium ion cells more suited to renewable energy storage, especially solar power.
Solar batteries are by nature only charged during the day and then discharged at night. The ideal off grid solar power system has a battery that is never fully empty but also never kept fully charged. If it did reach full charge, it would mean no more energy could be stored and so sunlight would be wasted. In that case, either the solar array is too big for the battery or the battery is too small for the array. In Winter, most solar batteries struggle to maintain anything close to full charge and may spend days or even weeks at low to middle charge levels.
As you can see, Lithium ion cells fit this model much better than Lead acid cells. Lead acid cells are quickly damaged by not being kept fully charged 24 hours a day. Lithium ion cells PREFER not to be kept fully charged for longest calendar life.
Next time I'll discuss cycle lifespan.
Or why not talk to us at Sustainables4U about your energy storage needs!
