Is there storage luggage available when having a long stopover between flights at Beijing Airport?

Yes, there are left luggage service center at PEK. Hereunder are the detailed information:
T1: The lelf-luggage counters is on the east of Corridor between T1 to T2
T2: The left-luggage counters are near gate 9 in the arrival hall on the first floor and gate 10 and gate 8 on the departure hall on the second floor.
T3: East of public space of Arrival Hall at 2nd foor of T3-C; West of public space of Arrival Hall at 2nd foor of T3-C

Hope helpful!